dement v.

"drive mad, bring to a state of dementia," now obsolete or archaic but for the past-participle adjective demented, 1540s, probably from 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 "from, away from" (see de)+ mente, ablative of mens "mind" (from PIE root *men- (1) "to think").