c. 1600, in law, "not capable, mentally, of managing one's own affairs," Latin, "not master of one's mind," from non "not" + compos "having power" (from com- "together" + potis "powerful") + mentis "of the mind," genitive of mens "mind."
Entries linking to non compos mentis
nincompoop n.
1670s, nicompoop; the modern form is attested by 1713. Despite the similarity [noted by Johnson] to the 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.
"And dost thou bid me good morrow ? Why, you Ninny, you Nicompoop, you Noun Adjective, for thou canst not stand by thy self, I am sure;" etc. [Thomas D'Urfey, "A Fool's Preferment," 1688]