Enough is native, and its gh is a fossil of a sound English no longer makes — except that here, unusually, the sound did not vanish. It hardened into an f. That is the other fate of Old English gh: sometimes it fell silent, as in night and daughter, and sometimes it toughened, as in enough, rough, tough and laugh. Same letters, two different outcomes, which is a large part of why -ough is such chaos.
The word is Old English genog, and its first element is a small piece of buried grammar. That ge- is the Old English prefix meaning "with, together," the equivalent of Latin com- and of the ge- still alive in German. It has otherwise vanished from English almost entirely, worn down and dropped, which makes enough its most prominent surviving example — a dead prefix hiding in an everyday word.
enough (adj.) c. 1300, from Old English genog "sufficient in quantity or number," from Proto-Germanic compound *ganog "sufficient" (source also of 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- ). The second element is from PIE *nok- , from root *nek- (2) "to reach, attain" (source also of Sanskrit asnoti "to reach," 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 , implying the end of discussion, is attested from 1839, American English, representing a casual or colloquial pronunciation. also from c. 1300
enough came in through Old English, and its spelling follows that family's conventions.
How that family writes these sounds
| a vowel that runs longer than it should, before nothing you can hear | gh — silent, or said as flight, night, daughter, enough |
What gives it away on the page
| gh | Stands for a throat sound (like the ch in Bach) that English later dropped or turned into f.light, thought, plough, enough |
Old English in the spelling guide · guess another word's family