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.
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.