“Crazy as a shithouse rat.” Why are shithouse rats crazy?
Never heard the expression. The only usage I’ve ever heard is “Cunning as a shithouse rat”. That makes perfect sense since shithouse rats were cunning creatures, they needed to be to avoid sudden death.
I assume this is just one more of those metaphors that have become jumbled through usage.
Well, using my usual tool for determining the currency of a particular phrase, “cunning as a shithouse rat” gets 212 Google hits, whereas “crazy as a shithouse rat” gets 600. But why would a shithouse rat be particularly cunning or crazy? Is it the methane gas?
Nah, the same reason foxes are considered cunning. Any vermin that is hard to trap, poison or otherwise kill will get a reputation for being cunning. A rat that you see every time you go to the shithouse, but that never takes bait from traps, is never there when you have a firearm and so forth will get a reputation for cunning. In essence the only live rats are cunning rats.
Interesting thought occurs to me, that foxes also gave rise to the expression “carzy like a fox”. I wonder if there hasn’t been some cross pollination between different metaphors concenring cunning?
I think it’s just a bizarre profanity, a bit of spicy language and not analogous to anything.
There are others, “fuckin’ A ditty bag” comes to mind, which will be familiar to old sailors and marines.
I always heard it as “shit house mouse”, which, at least, has a rhyming quality.
If you were a rat, would you choose to live in a shithouse? Only the crazy rats do.
You know, as a kid in southern Ontario, all I ever heard was “fat as a shithouse rat”.
Sorry; “fat as a shithouse rat” returns zero hits on Google. Clearly, you and your friends were aberrant. 
Well, it wasn’t my friends so much as my old man and big brother, who were of a generation who might actually have seen rats in outhouses.
It seems odd that my expression is the one with no cites…in some ways in makes more sense than the “crazy” one.
I’ve always heard the expression as, “crazier than a…” That might get more Google hits…
Yep. 10,300 to be exact.
I think this is part of the explanation. But it also allows the use of the words “crazy,” “shit,” and “rat,” all of which are individually amusing, together in the same expression, which makes it triply entertaining.
One that seems odder to me is “built like a brick shithouse,” which can mean a woman who is really stacked.
I might be wrong here, but I was under the impression that the original phrase was “nuttier than a shithouse rat,” which is a pun on “nuttier.”
… the pun being that nuts are undigested and show up in feces.
Yes. As aberrant as a shithouse rat explanation. 
Any headway on this? I’m curious to see if I’m right.
OOOOOOOOHHHH. Now I get it.
In your defense, your take on the saying is a popular one with the old men at the feed store who meet up at 4:00 am to drink Jack 'n coffee and play dominoes in Blue Ridge, TX.
I’ve known this phrase for ages, I like the shithouse mouse variation. I have always just thought of it as a particularly funny southern phrase.
Turns out, it was used in the movie “Stand By Me”
Milo: I know who you are. You’re Teddy Duchamp. Your dad’s a looney. A looney up in the nuthouse in Togus. He took your ear and he put it to a stove and burnt it off.
Teddy: My father stormed the beach at Normandy.
Milo: He’s crazier than a shithouse rat. No wonder you’re acting the way you are with a looney for a father.
Teddy: You call my dad a looney again, I’ll kill you.
Milo: Looney, looney, looney.
Teddy: Aah! I’m gonna rip your head off and shit down your throat!
ETA: I guess the real question is, was it in the book? Or was it dialogue made up for the movie.
I know this term from a type of “hooligan” fun song…
“shit-house rats, we are here, to shag your women and drink your beer”.
British perspective!