not know your arse from your elbow

Meaning

1. to be very stupid or completely lacking in skill

From the same entry

• before you know where you are
• be not to know
• for all you, I, they, know
• God knows
• I don't know how, why, etc.
• I know
• know as well as I do
• know backwards
• know best
• know better
• know by sight
• know different