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.
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