X
from French.
wiktionary
Introduced by René Descartes in the context of mathematical notation, but has since widened to other contexts.
X ( upper case, lower case x, plural Xsor X's)
Presumably by abbreviation of the pronunciation of ecstasy.
X (not comparable)
From Christ by abbreviation, orthographic borrowing from Ancient Greek Χ(Kh, letter chi), from Χριστός(Khristós, “ Christ”).
From cross, due to the X symbol being a cross saltire.
X ( comparative more X, superlative most X)
etymonline
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 (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.
x (v.)
"cross out with an 'X'," 1942, from X.
