It stands for "don't be a torch." If your friend is being lame you tell them this.
Danny: Can I borrow your Harry Potter Blu-ray?
Kevin: I charge $0.50 per disc.
Brian: My car broke down, can you drive us to the IMAX movie instead?
Kevin: Only if you pay me gas money.
Brian: DBAT, you don't even pay for your gas, your parents do.