"to drive mad," 1540s, probably from Middle French démenter, from Late Latin dementare "to drive out of one's mind," from stem of Latin demens "out of one's senses, insane, raving, foolish; distracting, wild, reckless" (a less technical term than insanitas), from phrase de mente, from de + mente, ablative of mens mind" (see mind (n.)).