French (adj.)youdaoicibaDictYouDict[French 词源字典]
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."[French etymology, French origin, 英语词源]
french (v.)youdaoicibaDictYouDict
"perform oral sex on," c. 1917, from French (adj.), reflecting Anglo-Saxon equation of Gallic culture and sexual sophistication. Related: Frenched; frenching.
French (n.)youdaoicibaDictYouDict
from Old English frencisc (early Middle English frencisc, frenscen) "French person; the French nation," from the adjective (see French (adj.)). From c. 1300 as "the French language." Euphemistic meaning "bad language" (pardon my French) is from 1895. French Français is from Medieval Latin *francencis, from francus "a Frank" + nationality suffix -ensis "belonging to" (see -ese).