Definition
I am not sure that I agree; expressing doubt or skepticism.
How People Actually Use It
A phrase that is slang for no, normally used when in relation to an activity
"Ex.1 Guy 1: aye, pass the bong Guy 2: I don't know about that one cheif ex.2 Guy 1: ay bruh, can you pull out? Guy 2: I don't know about that one chief"
Source: Urban Dictionary