X Look up X at Dictionary.com
The entire entry for X in Johnson's dictionary (1756) is: "X is a letter, which, though found in Saxon words, begins no word in the English language." Most English words beginning in -x- are of Greek origin or modern commercial coinages. East Anglian in 14c. showed a tendency to use -x- for initial sh-, sch- (such as xal for shall), which didn't catch on but seems an improvement over the current system. As a symbol of a kiss on a letter, etc., it is recorded from 1765. In malt liquor, XX denoted "double quality" and XXX "strongest quality" (1827).

Algebraic meaning "unknown quantity" (1660 in English, from French), sometimes is said to be from medieval use, originally a crossed -r-, in that case probably from Latin radix (see root (n.)). Other theories trace it to Arabic (Klein), but a more prosaic explanation says Descartes (1637) took x, y, z, the last three letters of the alphabet, for unknowns to correspond to a, b, c, used for known quantities.

Used allusively for "unknown person" from 1797, "something unknown" since 1859. As a type of chromosome, attested from 1902 (first so called in German; Henking, 1891). To designate "films deemed suitable for adults only," first used 1950 in Britain; adopted in U.S. Nov. 1, 1968. The XYZ Affair in American history (1797) involved French agents designated by those letters.
x (v.) Look up x at Dictionary.com
"cross out with an 'X'," 1942, from X.
X-ray (n.) Look up X-ray at Dictionary.com
1896, X-rays, translation of German X-strahlen, from X, algebraic symbol for an unknown quantity, + Strahl (plural Strahlen) "beam, ray." Coined 1895 by German scientist Wilhelm Conrad Röntgen (1845-1923), who discovered them, to suggest that the exact nature of the rays was unknown. As a verb by 1899. Meaning "image made using X-rays" is from 1934, earlier in this sense was X-radiograph (1899).
Xanadu Look up Xanadu at Dictionary.com
Mongol city founded by Kublai Khan, 1620s, Englished form of Shang-tu. Sense of "dream place of magnificence and luxury" derives from Coleridge's poem (1816).
xanthic (adj.) Look up xanthic at Dictionary.com
"yellowish," 1817, from French xanthique, from Greek xanthos "yellow" (see xantho-).
Xanthippe Look up Xanthippe at Dictionary.com
also (incorrectly) Xantippe, late 16c., spouse of Socrates (5c. B.C.E.), the prototype of the quarrelsome, nagging wife. The name is related to the masc. proper name Xanthippos, a compound of xanthos "yellow" (see xantho-) + hippos "horse" (see equine).
xantho- Look up xantho- at Dictionary.com
before vowels xanth-, word-forming element meaning "yellow," from Greek xanthos "yellow" of various shades; used especially of hair and horses, of unknown origin. Used in scientific words; such as xanthein (1857) "soluble yellow coloring matter in flowers," xanthophyll (1838) "yellow coloring matter in autumn leaves." Also Huxley's Xanthochroi (1867) "blond, light-skinned races of Europe" (with okhros "pale").
xanthosis (n.) Look up xanthosis at Dictionary.com
1857, Modern Latin, from Greek xanthos (see xantho-) + -osis.
xanthous (adj.) Look up xanthous at Dictionary.com
1829, "fair-haired and light-complexioned," from Greek xanthos "yellow," of unknown origin (see xantho-). But the word also was used in 19c. anthropology as "specifying the yellow or Mongoloid type of mankind" [Century Dictionary].
xebec (n.) Look up xebec at Dictionary.com
"small three-masted vessel," favored by Barbary corsairs but also used in Mediterranean trade, by 1745, from French chébec, from Italian sciabecco, ultimately from Arabic shabbak "a small warship." Altered by influence of cognate Spanish xabeque, which shows the old way of representing the Spanish sound now spelled -j-.
xenelasia (n.) Look up xenelasia at Dictionary.com
"prevention of aliens from settling in Sparta," Greek, literally "expulsion of foreigners," from xenelatein "to expel foreigners," from xenos "stranger" (see xeno-) + elatos, verbal adjective of elaunein "drive, drive away, beat out."
Xenia Look up Xenia at Dictionary.com
city in Ohio, from Greek xenia "hospitality, rights of a guest, friendly relation with strangers," literally "state of a guest," from xenos "guest" (see guest (n.)). Founded 1803 and named by vote of a town meeting, on suggestion of the Rev. Robert Armstrong to imply friendliness and hospitality.
xenial (adj.) Look up xenial at Dictionary.com
"pertaining to hospitality," 1834, from Greek xenia (see Xenia) + -al (1). Related: Xenially.
xeno- Look up xeno- at Dictionary.com
before vowels, xen-, word-forming element meaning "strange, foreign; stranger, foreigner," from Greek xeno-, comb. form of xenos "a guest, stranger, foreigner, refugee, guest-friend, one entitled to hospitality," cognate with Latin hostis (see guest (n.)). "The term was politely used of any one whose name was unknown" [Liddell & Scott].
xenogamy (n.) Look up xenogamy at Dictionary.com
"fertilization by pollen from a different plant," 1877, from xeno- "strange, foreign" + -gamy "fertilization." Related: Xenogamous.
xenolith (n.) Look up xenolith at Dictionary.com
1894, from xeno- "foreign, strange" + -lith "stone."
xenon (n.) Look up xenon at Dictionary.com
gaseous element, 1898, from Greek xenon, neuter of xenos "foreign, strange" (see xeno-); coined by its co-discoverer, Scottish chemist Sir William Ramsay (1852-1916); compare krypton.
xenophile (nj.) Look up xenophile at Dictionary.com
1922, from xeno- "foreign, strange" + -phile.
xenophilic (adj.) Look up xenophilic at Dictionary.com
1974, from xenophile + -ic.
xenophobe (n.) Look up xenophobe at Dictionary.com
1897, from xeno- "foreign, strange" + -phobe. As an adjective from 1908.
xenophobia (n.) Look up xenophobia at Dictionary.com
1903, from xeno- "foreign, strange" + -phobia "fear." Earlier (c. 1884) it meant "agoraphobia."
xenophobic (adj.) Look up xenophobic at Dictionary.com
1912, from xenophobia + -ic.
xerasia (n.) Look up xerasia at Dictionary.com
"excessive dryness of hair," 1706, medical Latin, from Greek xerasia "dryness," from xeros "dry, withered," from PIE *ksero- "dry."
Xeres Look up Xeres at Dictionary.com
Andalusian town (modern Jerez) famous for its wine; see sherry. For first letter, see xebec.
xeric (adj.) Look up xeric at Dictionary.com
1926; see xero- + -ic.
xero- Look up xero- at Dictionary.com
before vowels, xer-, word-forming element meaning "dry," from Greek xero-, comb. form of xeros "dry, withered" (see xerasia).
xeroderma (n.) Look up xeroderma at Dictionary.com
1848, from xero- + derma.
xerography (n.) Look up xerography at Dictionary.com
"photographic reduplication without liquid developers," 1948, from Greek xeros "dry" (see xerasia) + -ography as in photography. Related: Xerographic.
xerophagy (n.) Look up xerophagy at Dictionary.com
"habit of living on dry food," especially as a form of fasting, 1650s, from xero- + -phagy (see -phagous).
xerophilous (adj.) Look up xerophilous at Dictionary.com
"drought-loving," 1863, from xero- + -philous, from Greek from philos "loving," of uncertain origin.
xerophyte (n.) Look up xerophyte at Dictionary.com
1897, from xero- + Greek phyton "a plant" (see phyto-).
xerosis (n.) Look up xerosis at Dictionary.com
1890, Modern Latin, from Greek xerosis, from xeros "dry" (see xerasia) + -osis.
xerotic (adj.) Look up xerotic at Dictionary.com
"characterized by dryness," 1901, from stem of xero- + -ic.
Xerox Look up Xerox at Dictionary.com
1952, trademark taken out by Haloid Co. of Rochester, N.Y., for a copying device, from xerography. The verb is first attested 1965, from the noun, despite strenuous objection from the Xerox copyright department. Related: Xeroxed; Xeroxing.
Xerxes Look up Xerxes at Dictionary.com
king of Persia who reigned 486-465 B.C.E., Greek Xerxes, from Old Persian Xšayaršan, literally "male (i.e. 'hero') among kings," from Xšaya- "to rule over" (see shah) + aršan "male, man, hero." The Hebrew rendition was Ahashwerosh, Ahashresh.
Xhosa (n.) Look up Xhosa at Dictionary.com
South African Bantu people, 1801, their self-designation. Also of their language.
xi (n.) Look up xi at Dictionary.com
fourteenth letter of the Greek alphabet.
xiphias (n.) Look up xiphias at Dictionary.com
1660s, genus of swordfish, from Greek xiphias "swordfish," from xiphos "a sword" (see xiphoid). Related: Xiphioid.
xiphoid (adj.) Look up xiphoid at Dictionary.com
"sword-shaped," 1746, from Greek xiphos "a sword," of unknown origin (Klein suggests a Semitic source and compares Hebrew sayif, Arabic sayf) + -oid.
Xmas (n.) Look up Xmas at Dictionary.com
"Christmas," 1551, X'temmas, wherein the X is an abbreviation for Christ in Christmas, English letter X being identical in form (but not sound signification) to Greek chi, the first letter of Greek Christos "Christ" (see Christ). The earlier way to abbreviate the word in English was Xp- or Xr- (corresponding to the "Chr-" in Greek Χριστος), and the form Xres mæsse for "Christmas" appears in the "Anglo-Saxon Chronicle" (c. 1100).
xylem (n.) Look up xylem at Dictionary.com
"woody tissue in higher plants," 1875, from German Xylem, coined from Greek xylon "wood" (see xylo-).
xylene (n.) Look up xylene at Dictionary.com
1851, from Greek xylon "wood" (see xylo-) + -ene.
xylo- Look up xylo- at Dictionary.com
before vowels xyl-, word forming element meaning "wood," from comb. form of Greek xylon "wood cut and ready for use, firewood, timber; piece of wood; stocks, a plank, beam, or bench," in New Testament, "the Cross," of unknown origin.
xylophagous (adj.) Look up xylophagous at Dictionary.com
1842, from Latinized form of Greek xylophagos "wood-eating;" see xylo- + -phagous.
xylophone (n.) Look up xylophone at Dictionary.com
1866, coined from Greek xylon "wood" (see xylo-) + phone "a sound," from PIE root *bha- (2) "to speak, tell, say" (see fame (n.)).
xyster (n.) Look up xyster at Dictionary.com
"surgical instrument for scraping bones," 1680s, from Greek xyster "a graving tool," from xyein "to scrape," from PIE root *kes- (1) "to scrape."