you ain’t know, phr. — Green’s Dictionary of Slang

you ain’t know phr.

[i.e. you didn’t know that already?]

(US black/ campus) a phr. of affirmation, esp. when confirming what the speaker seems only to be questioning.

1996		Eble Campus Sl. Mar. 9: you ain’t know – expression of interest, sometimes agreement: X: ‘That concert was really good.’ Y: ‘You ain’t know.’.
2000		Ebonics Primer at www.dolemite.com 🌐 You ain’t know? Definition: ‘No shit’ or ‘You are not aware of this?’ Example: Man, dat’s a fine car you got. Reply: You ain’t know?