"extremely low condition of mental function, mental incapacity," 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. Especially in reference to senile dementia "the failure of mind which occurs in old age" (1822). Dementia praecox for what now would be called schizophrenia is a Modern Latin form recorded from 1899 in English, 1891 in German, from French démence précoce (1857). See precocious.