Wiktionary
n. (context UK slang English) an insane person; a nutcase
WordNet
n. a person suffering from neurosis [syn: neurotic, psychoneurotic]
Usage examples of "mental case".
Not that Harry was a mental case, but it had provided an easy method of bringing him under Anderson's control without the Necroscope himself knowing what was happening.
Even allowing for all that, if only Gus hadn't painted a picture of a man-hating mental case .
If he could have sold the mental case something he would gladly have accepted the South African coin.
Get rid of Mercy, she's a nutcase anyway if you ask me, a right mental case.
Then Sheng will get the message that his conspiracy's known to us and he'll get it unofficially, not from us but from an unsanctioned mental case.
He had the desperate look of a man pushed too fast, too far: like a drunk or a mental case.
He had wasted time and fuel on an idiot who was no more than a normal mental case after all.