Online Etymology Dictionary

X: 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.): "cross out with an 'X'," 1942, from X.
X-ray (n.): 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: 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.): "yellowish," 1817, from French xanthique, from Greek xanthos "yellow" (see xantho-).
Xanthippe: 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-: 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.): 1857, Modern Latin, from Greek xanthos (see xantho-) + -osis.
xanthous (adj.): 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.): "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.): "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: 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.): "pertaining to hospitality," 1834, from Greek xenia (see Xenia) + -al (1). Related: Xenially.
xeno-: 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.): "fertilization by pollen from a different plant," 1877, from xeno- "strange, foreign" + -gamy "fertilization." Related: Xenogamous.
xenolith (n.): 1894, from xeno- "foreign, strange" + -lith "stone."
xenon (n.): 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.): 1922, from xeno- "foreign, strange" + -phile.
xenophilic (adj.): 1974, from xenophile + -ic.
xenophobe (n.): 1897, from xeno- "foreign, strange" + -phobe. As an adjective from 1908.
xenophobia (n.): 1903, from xeno- "foreign, strange" + -phobia "fear." Earlier (c. 1884) it meant "agoraphobia."
xenophobic (adj.): 1912, from xenophobia + -ic.
xerasia (n.): "excessive dryness of hair," 1706, medical Latin, from Greek xerasia "dryness," from xeros "dry, withered," from PIE *ksero- "dry."
Xeres: Andalusian town (modern Jerez) famous for its wine; see sherry. For first letter, see xebec.
xeric (adj.): 1926; see xero- + -ic.
xero-: before vowels, xer-, word-forming element meaning "dry," from Greek xero-, comb. form of xeros "dry, withered" (see xerasia).
xeroderma (n.): 1848, from xero- + derma.
xerography (n.): "photographic reduplication without liquid developers," 1948, from Greek xeros "dry" (see xerasia) + -ography as in photography. Related: Xerographic.
xerophagy (n.): "habit of living on dry food," especially as a form of fasting, 1650s, from xero- + -phagy (see -phagous).
xerophilous (adj.): "drought-loving," 1863, from xero- + -philous, from Greek from philos "loving," of uncertain origin.
xerophyte (n.): 1897, from xero- + Greek phyton "a plant" (see phyto-).
xerosis (n.): 1890, Modern Latin, from Greek xerosis, from xeros "dry" (see xerasia) + -osis.
xerotic (adj.): "characterized by dryness," 1901, from stem of xero- + -ic.
Xerox: 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: 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.): South African Bantu people, 1801, their self-designation. Also of their language.
xi (n.): fourteenth letter of the Greek alphabet.
xiphias (n.): 1660s, genus of swordfish, from Greek xiphias "swordfish," from xiphos "a sword" (see xiphoid). Related: Xiphioid.
xiphoid (adj.): "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.): "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.): "woody tissue in higher plants," 1875, from German Xylem, coined from Greek xylon "wood" (see xylo-).
xylene (n.): 1851, from Greek xylon "wood" (see xylo-) + -ene.
xylo-: 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.): 1842, from Latinized form of Greek xylophagos "wood-eating;" see xylo- + -phagous.
xylophone (n.): 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.): "surgical instrument for scraping bones," 1680s, from Greek xyster "a graving tool," from xyein "to scrape," from PIE root *kes- (1) "to scrape."