c.1300, from Old English genog, a common Germanic formation (cf. Old Saxon ginog, Old Frisian enoch, Dutch genoeg, Old High German ginuog, German genug, Old Norse gnogr, Gothic ganohs).
This is a compound of ge- "with, together" (also a participial, collective, intensive, or perfective prefix) + root -nah, from PIE *nek- "reach, attain" (cf. Sanskrit asnoti "reaches," Hittite ninikzi "lifts, raises," Lithuanian nešti "to bear, carry," Latin nancisci "to obtain").
It is the most prominent among the surviving examples of Old English