French (adj.)youdaoicibaDictYouDict
c. 1200, frensh, frenche, "pertaining to France or the French," from Old English frencisc "French," originally "of the Franks," from franca, the people name (see Frank). A similar contraction of -ish is in Dutch, Scotch, Welsh, suggesting the habit applies to the names of only the intimate neighbors.

In some provincial forms of English it could mean simply "foreign." Used in many combination-words, often dealing with food or sex: French dressing (by 1860); French toast (1630s); French letter "condom" (c. 1856, perhaps on resemblance of sheepskin and parchment), french (v.) "perform oral sex on," and French kiss (1923) all probably stem from the Anglo-Saxon equation of Gallic culture and sexual sophistication, a sense first recorded 1749 in the phrase French novel. (In late 19c.-early 20c., a French kiss was a kiss on each cheek.) French-Canadian is from 1774; french doors is by 1847. To take French leave, "depart without telling the host," is 1771, from a social custom then prevalent. However, this is said to be called in France filer à l'anglaise, literally "to take English leave."
XyoudaoicibaDictYouDict
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.