not know your arse from your elbow
idiomPart ofelbow
not know your arse from your elbow — idiom
1. To be very stupid or lack basic skill or knowledge.
1.idiom
Definition
To be very stupid or lack basic skill or knowledge.
Example
That mechanic doesn't know his arse from his elbow.
How can she be a doctor if she doesn't know her arse from her elbow?