can't tell (your) arse from (your) elbow

very informal!, very informal!

if you can't tell your arse from your elbow, you are stupid and become confused about simple things.

It's no good asking him to organize anything - he can't tell his arse from his elbow.

• cost an arm and a leg
• cost a pretty penny
• cost dear
• cost someone an arm and a leg
• cost someone a pretty penny
• cost someone dear
• cotton-picking
• cotton to
• cotton to someone
• cotton to something
• couch potato
• cough up
• cough up something
• could do something in sleep
• could do something in their sleep
• could do something standing on head
• could do something standing on your head
• could do something with eyes closed
• could do something with eyes shut
• could do something with one arm tied behind back
• could do something with one arm tied behind their back
• could do something with one hand tied behind back
• could do something with one hand tied behind their back
• could do something with their eyes closed
• could do something with their eyes shut