word: nincompoop

The Oxford English Dictionary lists the origins as unknown. But was it a medieval insanity plea?

The OED professes to be stumped over this one, which is odd, because the word so closely resembles the medieval Latin legal phrase, non compos mentis. According to Wikipedia:

The status of non compos mentis applied to those who were not mad from birth, but became so later in life through no fault of their own. The property and interests of such a person could be committed to another party to conserve and administer for the duration of the madness. This contrasted with "natural fools" who were mad from birth and whose property interests passed to the crown, and habitual drunkards, who could claim no defense of madness.

Non compos mentis was actually the most favorable form of legal insanity. Nincompoops were not born insane. They could still owned stuff. They just had to submit to somebody else’s care, like The Girl with the Dragon Tattoo or Britney Spears.

Non compos mentis > Non compos > Nincompoop

I rest my case.