Etymology
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French (adj.)

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."

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distinguish (v.)

1560s, "recognize as different or distinct from what is contiguous or similar; perceive, make out," from French distinguiss-, stem of distinguer, or directly from Latin distinguere "to separate between, keep separate, mark off, distinguish," perhaps literally "separate by pricking," from assimilated form of dis- "apart" (see dis-) + -stinguere "to prick" (compare extinguish and Latin instinguere "to incite, impel").

Watkins says "semantic transmission obscure;" the sense might be from "pricking out" as the old way to make punctuation in parchment or some literal image, but de Vaan derives the second element from a different PIE root meaning "to push, thrust":

The meanings of ex- and restinguere 'to extinguish' and distinguere seem quite distinct, but can be understood if the root meant 'to press' or 'push': ex-stinguere 'to put a fire out', re-stinguere 'to push back, suppress', and dis-stinguere 'to push apart [thence] distinguish, mark off ....

The suffix -ish is due to the influence of many verbs in which it is the equivalent of Old French -iss-, ultimately from Latin inchoative suffix -iscere (this is also the case in extinguish, admonish, and astonish).

Sense of "to mark or note in a way to indicate difference" is from 1570s; that of "separate from others by some mark of honor or preferment" is from c. 1600. Intransitive meaning "make a distinction, find or show difference (between)" is from 1610s. Related: Distinguishing. The Middle English form of the verb was distinguen (mid-14c.).

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