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general::
Phrase(s): doesn’t know his ass from a hole in the ground [and] doesn’t know his ass from his elbow
doesn’t know anything; acts ignorant. (Use ass with caution. Also with don’t. Don’t used with all persons is folksy.) • That teacher doesn’t know his ass from a hole in the ground. • She’s supposed to be an expert, but she doesn’t know her ass from her elbow.
McGrawhill's American Idioms And Phrasal Verbs