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.
See also: arse, elbow, know
Farlex Dictionary of Idioms.

not know your arse from your elbow

BRITISH, INFORMAL, VERY RUDE or

not know your ass from your elbow

mainly 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.
See also: arse, elbow, know, not
Collins COBUILD Idioms Dictionary, 3rd ed.

not know your arse from your elbow

be totally ignorant or incompetent.
See also: arse, elbow, know, not
Farlex Partner Idioms Dictionary

not know your ˌarse from your ˈelbow

(British English) (American English not know your ˌass from your ˈelbow) (taboo, slang) be very stupid or completely lacking in skill: Don’t ask him to organize things! He doesn’t know his arse from his elbow!
See also: arse, elbow, know, not
Farlex Partner Idioms Dictionary
See also:
  • be as brown as a berry
  • berry
  • (as) brown as a berry
  • as brown as a berry
  • a turn of phrase
  • a plain Jane
  • a piece of the action
  • a piece/slice of the action
  • appear to
  • a change of heart