doesn't/wouldn't know (one's) ass from (one's) elbow

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doesn't/wouldn't know (one's) ass 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 doesn't know her ass from her elbow.
See also: ass, elbow, know
Farlex Dictionary of Idioms.
See also:
  • a plain Jane
  • appear to
  • a change of heart
  • a shoulder to cry on
  • a mystery to (one)
  • be (on) the wrong side of (an age)
  • be on the right/wrong side of 40, 50, etc.
  • a turn of phrase
  • able to do it