can't tell (one's) arse from (one's) elbow

(redirected from can't tell their arses from their elbows)
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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.
See also: arse, elbow, tell