"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."
Similar to a snake in the grass, a snake in the ground is a far snakier snake. They are much harder to spot than their grass-dwelling counterparts and willstrike when you least expect it.
"Hey man, keep an eye on that tall grass, could be a snake hidin' there."
"Hell, I ain't worried about some boring grass snake. There could be a snake in the ground right under our goddamn feet!"
"Damn man, you know that Becky my girl's been hanging with?"
"Yeah, Becky, she that older lady right?"
"Shit, it turns out she done Mrs. Doubtfired my ass. Revealed herself to be a man and made off with my girl."
"What a real snake in the ground."