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 (see chi) 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.

"cross out with an 'X'," 1942, from X.

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).

"yellowish," 1817, from French xanthique, from Greek xanthos "yellow" (see xantho-).

1857, Modern Latin, from Greek xanthos (see xantho-) + -osis.

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].

"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-.

"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."

"pertaining to hospitality," 1834, from Greek xenia (see Xenia) + -al (1). Related: Xenially.

"fertilization by pollen from a different plant," 1877, from xeno- "strange, foreign" + -gamy "fertilization." Related: Xenogamous.