Etymology

A Latin phrase coined in German (by Arnold Pick in 1891) and later adopted into English: dēmentia ‎(“madness”, “insanity”) + praecox ‎(“premature”, “untimely”) (whence the English precocious) = “untimely madness” ≈ “premature dementia” (here, dēmentia carries the connotation of senile dementia).