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1.
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.
by pdreezy October 19, 2008
 
2.
A lowly rodent who, out of desperation, has learned to live alongside man by occupying one of our less-than-ideal structures, the outhouse.

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.
After being denied $20 from the ATM machine for lack of funds: "Damnit, I'm poorer than a shithouse mouse"
by Digital Logic May 27, 2003
 
3.
An exclamation of frustration
I guess my computer has decided to stop having a "turn off" option. Well, shithouse mouse, I'll just have to let the battery run down.
by TFill January 06, 2012