noun 1. a mentally ill person (not in technical use). an extremely foolish or eccentric person. "this lunatic just accelerated out of the side of the road" synonyms: maniac, madman, madwoman, imbecile, psychopath, psychotic; fool, idiot; eccentric; informalloony, nut, nutcase, nutjob, head case, psycho, moron, screwball, crackpot, fruitcake, fruit loop, loon "he drives like a lunatic". Middle English: from Old French lunatique, from late Latin lunaticus, from Latin luna ‘moon’ (from the belief that changes of the moon caused intermittent insanity).
