The Collaborative International Dictionary
Non compos \Non com"pos\ Non compos mentis \Non com"pos men"tis\ Not of sound mind; not having the regular use of reason; hence, also, as a noun, an idiot; a lunatic; one devoid of reason, either by nature or from accident.
Douglas Harper's Etymology Dictionary
Latin, literally "not master of one's mind."
Wiktionary
a. (context legal lang=en but also seeing wider use) insane; not of sound mind; not in control of one's self.
WordNet
adj. not of sound mind, memory, or understanding; in law, not competent to go to trial [syn: non compos mentis(p), of unsound mind(p)]
Wikipedia
Non compos mentis (latin) is the legal term meaning "not of sound mind". Non compos mentis derives from the Latin non meaning "not", compos meaning "having command" or "composed", and mentis ( genitive singular of mens), meaning "of mind". It is the direct opposite of Compos mentis (of a sound mind).
Usage examples of "non compos mentis".
How can you do or say anything calling for an apology when you are legally non compos mentis?
The fact that he is clearly non compos mentis will carry no weight with the Prince when he gets wind of this.
But the old Latin joke came back to him: Penis erectus non compos mentis.
I went to Felix's parties and I trucked with Georgie once, and Felix thought I was non compos mentis, so if he sent you to see me he was probably playing a game.
He was killed because he had for too long made G-men, all of whom then wore fedoras, look non compos mentis, like nincompoops.