enoughyoudaoicibaDictYouDict[enough 词源字典]
enough: [OE] Enough is a widespread word in the Germanic languages. German has genug, Dutch genoeg, and Swedish and Danish the reduced forms nog and nok. All go back to a prehistoric Germanic *ganōgaz, a compound formed from the collective prefix *ga- and an Indo-European element *nak- whose underlying meaning is probably ‘reach, attain’ (it occurs in Sanskrit na and Latin naniscī, both of which have that sense).
[enough etymology, enough origin, 英语词源]
enough (adj.)youdaoicibaDictYouDict
c. 1300, from Old English genog "sufficient in quantity or number," from Proto-Germanic compound *ganoga- "sufficient" (cognates: Old Saxon ginog, Old Frisian enoch, Dutch genoeg, Old High German ginuog, German genug, Old Norse gnogr, Gothic ganohs).

First element is Old English ge- "with, together" (also a participial, collective, intensive, or perfective prefix), making this word the most prominent surviving example of the Old English prefix, the equivalent of Latin com- and Modern German ge- (from PIE *kom- "beside, near, by, with;" see com-). Second element is from PIE *nok-, from root *nek- (2) "to reach, attain" (cognates: Sanskrit asnoti "reaches," Hittite ninikzi "lifts, raises," Lithuanian nešti "to bear, carry," Latin nancisci "to obtain").

As an adverb, "sufficiently for the purpose," in Old English; meaning "moderately, fairly, tolerably" (good enough) was in Middle English. Understated sense, as in have had enough "have had too much" was in Old English (which relied heavily on double negatives and understatement). As a noun in Old English, "a quantity or number sufficient for the purpose." As an interjection, "that is enough," from c. 1600. Colloquial 'nough said is attested from 1839.