You aint Down 

when someone tells you this, they are trying to say that you can't do something cuz you're not brave. so you have to reply "i'm down", and then do it

guy 1: hey, are you down to go steal that car
guy 2: you aint down
guy 1: i'm down.....just watch

You aint Down by El Coyote June 26, 2008

You Ain't Down 

It's what your freinds tell you when you aren't ready to go or do something.

Nick: We out to lunch
Paul: I don't know...
Nick: You Ain't Down!

You Ain't Down by You Ain't Down December 17, 2008