Green’s Dictionary of Slang

does a bear shit in the woods? Is the pope (a) Catholic? phr.

also do beavers piss on flat rocks? does a bird have wings? does a chicken have lips? do I know my grandmother? do sheep wear sweaters? has a dog a nose? is the pope a guinea/polack?
[see Maledicta I:1 (Summer 1977) pp.77–82 for discussion of these ‘sarcastic interrogative affirmatives and negatives’]

[1910s+] (orig. US) a rhetorical phr. of which the implication is, ‘Don’t ask me stupid questions. Of course... .’ The phr. is also found reversed, ‘Does the pope shit in the woods? Is a bear a Catholic?’; note ad hoc vars. also included below, e.g. cit. 1914 (cf. can a duck swim? under duck n.1 ).