can't tell arse from elbow

can't tell your arse from your elbow

  (very informal!) also not know your arse from your elbow (very informal!)
if you can't tell your arse from your elbow, you are stupid and become confused about simple things It's no good asking him to organize anything - he can't tell his arse from his elbow.
See also: arse, elbow, tell
Full browser ?