nincompoop (n.)

1670s, nicompoop; modern form from 1713. Despite similarity [noted by Johnson] to Latin legal phrase non compos mentis "insane, mentally incompetent" (c. 1600), the connection is denied by the OED's etymologists because the earliest forms lack the second -n-. Weekley thinks first element may be a proper name, and cites Nicodemus, which he says was used in French for "a fool," or Nicholas. Klein says it is probably an invented word. Century Dictionary has no objection to the non compos mentis theory.