"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."
a town (esp. in prairie land) that has about as much going for it as an actual hole in the ground;
a boring, isolated, or otherwise unpleasant place or town;
an unremarkable, rural community.
cf. "shit hole," "backwater," "middle of bumfuck nowhere"
"Why would you live in Watrous, Saskatchewan? Place is a hole in the ground."
"I'm not trying to pick on Watrous, Saskatchewan, but it's a real hole in the ground."