Origin of the word X

1. most Eng. words beginning in -x- are of Gk. origin or modern commercial coinages. E. Anglian in 14c. showed a tendency to use -x- for initial sh-, sch- (cf. 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 Eng.), sometimes said to be from medieval use, originally a crossed -r-, probably from L. radix (see root). Other theories trace it to Arabic, 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 used 1950 in Britain to designate "films deemed suitable for adults only;" adopted in U.S. Nov. 1, 1968.

via Online Etymology Dictionary, ©2001 Douglas Harper