Not know your arse from your elbow
Definition 1 of 1
Pro Tip 1/3Informal usage
This phrase is very informal and often considered rude or unprofessional. 
In a casual conversation, someone might laugh and say, 'I don't know my arse from my elbow in the kitchen.'
Pro Tip 2/3UK expression
Predominantly used in the UK, this phrase may not be well understood elsewhere. 
When I studied in the UK, I heard someone use 'doesn't know his arse from his elbow' and had to ask what it meant.
Pro Tip 3/3Emphasizing ignorance
The phrase is used to strongly emphasize a person's lack of knowledge in a specific area. 
He tried to fix the car himself and ended up breaking it more—he clearly doesn't know his arse from his elbow.