know one's ass from a hole in the ground
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not know (one's) ass from a hole in the ground
rude slang To be exceptionally stupid, naïve, or dimwitted. I'm not surprised to hear that she got lost two blocks away from her house—she doesn't know her ass from a hole in the ground. Our prime minister wouldn't know his ass from a hole in the ground, so why do we think that he'll handle this negotiation with any amount of success?
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
know one’s ass from a hole in the ground
tv. to be knowledgeable; to be alert and effective. (Usually objectionable.) That stupid son of a bitch doesn’t know his ass from a hole in the ground.
McGraw-Hill's Dictionary of American Slang and Colloquial Expressions Copyright © 2006 by The McGraw-Hill Companies, Inc. All rights reserved.