not know your arse from your elbow
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doesn't/wouldn't know (one's) arse from (one's) elbow
rude slang Is very stupid or dimwitted. Primarily heard in UK. I'm not surprised to hear that she got lost two blocks away from her house—she doesn't know her arse from her elbow.
not know your arse from your elbowBRITISH, INFORMAL, VERY RUDE or
not know your ass from your elbowmainly AMERICAN, INFORMAL, VERY RUDE
If someone doesn't know their arse from their elbow, they are stupid or they do not know anything about a subject or situation. I won't be lectured to by some producer who doesn't know his arse from his elbow. Note: In British English, your arse is your bottom. In American English, the word used is ass.