can't tell (one's) arse from (one's) 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.
See also: arse, elbow, tell
Farlex Dictionary of Idioms.
- have nothing between the/(one's) ears
- rockhead
- not have much between the ears