What word does your mother always say?
Old English forgietan, from for-, used here with negative force, "away, amiss, opposite" + gietan "to grasp" (see get). To "un-get," hence "to lose" from the mind. A common Germanic construction (cf. Old Saxon fargetan, Old Frisian forjeta, Dutch vergeten, Old High German firgezzan, German vergessen "to forget"). The literal sense would be "to lose (one's) grip on," but that is not recorded in any Germanic language. Related: Forgetting; forgot; forgotten.