"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.
1. Statement of independance; can take care of own salon needs, etc.
2. Can be associated with "You don't know me" - assuming false pretense to understanding the situation and/or person; can be followed by a booteh bump and a finger snap
Some guy: I'll tell you a big fact.
Me: And... what is it?
That very guy: Stars don't blink! It's the atmosphere that makes stars look like if they blink!
Me: You don't say?