Acronym for "Don't Be a Dick;" a sacred code of conduct among honorable male circles. Much like the ancient concept of chivalry, but more intrinsic and fundemental.
Matt: Hey, can you turn it down? it's, like, 3 a.m. and I have a test at 8!
Sean: hell no!
Matt: dude, DBD
Sean: aw yeah, sorry dude. go get some sleep. *music stops*