Basically means you lying about something that you do or don't do. Another word for fronting, saying you have something that you don't have, or that you do something you don't really do, etc. Back in the day, we used to call in perpin, perpatrating, etc.
A girl who says she don't fuck on the first date, when everyone knows that she usually does, that's body bluffin'
A dude who pushing a big milky benz like it's his, when in reality, he's renting it, that's body bluffin'
For more examples, please see the Papoose Song "Body Bluffin'".