What is the X in X-mas?
1670s, nicompoop. Despite similarity [noted by Johnson] to Latin legal phrase non compos mentis "insane, mentally incompetent" (c.1600), the connection is denied by 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 probably an invented word.
A fool or stupid person (1676+)