ait (v.)

c. 1200, “to watch with hostile intent, lie in wait for, plot against,” from Anglo-French and Old North French waitier “to watch” (Old French gaitier “defend, watch out, be on one’s guard; lie in wait for;” Modern French guetter), from Frankish *wahton or another Germanic source, from Proto-Germanic *waht- (source also of Dutch wacht “a watching,” Old High German wahten, German wachten “to watch, to guard;” Old High German wahhon “to watch, be awake,” Old English wacian “to be awake”), from PIE root *weg- “to be strong, be lively.” General sense of “remain in some place” is from late 14c.; that of “to see to it that something occurs” is late 14c. Meaning “to stand by in attendance on” is late 14c.; specific sense of “serve as an attendant at a table” is from 1560s. Related: Waited; waiting.

From on-line etymology dictionary (Oxford)