Old English frencisc "of the Franks," from Franca (see Frank). The noun is from Old English Frencisc. As the name of a language, from late 13c.
Euphemistic meaning "bad language" (pardon my French) is from 1895. Used in many combination-words, often dealing with food or sex. French dressing recorded by 1860; French toast is from 1630s. French letter "condom" (c.1856, perhaps on resemblance of sheepskin and parchment), French (v.) "perform oral sex on" (c.1917) 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.
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."
Cunnilingus or fellatio; the FRENCH WAYv: Then the perverse chap actually Frenched her! (1917+)