Doesn't/wouldn't know (one's) ass from (one's) elbow American vulgar slang
"Ass" can be replaced by "arse" and "not know (one's) arse from (one's) elbow" is primarily heard in the UK.
Is extremely stupid
She is always getting lost on the way back home. She wouldn't know her ass from her elbow.
He constantly lost the key, so we can not enter the house. He doesn't know his ass from his elbow.
Extremely stupid
Very stupid, or silly
Stupid or crazy
(n): a fool person,
(adj): stupid, foolish, silly
To be very wet
Because of forgetting bringing an umbrella, I look like a drowned rat when it rains.