can't tell (one's) arse from (one's) elbow(redirected from can't tell their arse from their elbow)
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can't tell (one's) arse from (one's) elbow
rude slang Is very stupid or dimwitted. I'm not surprised to hear that she got lost two blocks away from her house—she's can't tell her arse from her elbow.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.