1806, from Latin dementia "madness, insanity," literally "a being out of one's mind," from dement-, stem of demens "mad, raving" (see dement) + abstract noun suffix -ia. It existed earlier in an Englished form, demency (1520s), from French démence. Dementia praecox is a Modern Latin form recorded from 1899 in English, 1891 in German, from French démence précoce (1857). See precocious.
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