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."
I saw the born-again Christians picketing at the train station again yesterday, every one of them crazier'na shithouse mouse.
Eternally mocked by the rest of the animal kingdom for it's choice of habitat, it passes through life barely noticed by man except in this phrase: "Poorer than a shithouse mouse"
Clearly indicating that the individual described does not even possess the meager funds required to reside in an outhouse.