An incomprehensibly crazy person.
NB: Not used to describe a spell of psychotic behavior. A shithouse mouse is permanently insane.
Used as a comparative standard, usually vis-a-vis actual insanity.
The term originated as a more mellifluous alternative to the term shithouse rat
; e.g., "as crazy as a shithouse rat."
Did you see Shavonne flip out and claw the skin off Joey Bag's arm last night? I'm glad you dumped that psycho, she's crazier than a shithouse mouse!
I saw the born-again Christians picketing at the train station again yesterday, every one of them crazier'na shithouse mouse.