From Latin 'demens' (out of one's mind) — 'de-' (away) + 'mens' (mind). A demented person has been driven from reason.
Suffering from dementia; driven mad; wildly insane or irrational.
From Latin dēmēns (out of one's mind, mad, insane, reckless), composed of dē- (away from, down from, reversing a state) + mēns (mind, reason, intention). The PIE root underlying mēns is *men- (to think), one of the most productive roots in the Indo-European family. It underlies Sanskrit manas (mind), Greek menos (spirit, mental force) and mania (madness), Latin mens (mind), memini (I remember), and monēre (to warn, to remind), as well as English