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you don't know your ass from a hole in the ground 

Figure of meaning that someone doesn't know what they're doing.

you don't know your ass from a hole in the ground 

"You don't know your ass from a hole in the ground." Illustrate with a pen and paper drawing a circle and a cracked circle. Now ask which is your ass and which is the hole in the ground. Subject points to cracked circle and you mock while pointing to their real derriere.
After listening to untruthful gibberish, you say to person, "you don't know your ass from a hole in the ground."

you dont know your ass from a hole in the ground 

you know nothing or you dont know what you are doing
sir you dont know your ass from a hole in the ground