FDB means "Fuck Dem Bitches." It's main use is to show one's disapproval of a group of bitches (women). This is not to be mistaken with "Fuck Dat Bitch," which is an alternate way to use FDB, which only applies to a single bitch (woman).
Mainly, if you are angry, upset or unhappy with a bitch or bitches, use this phrase to express yourself.
Guy 1: Dude, why haven't those girls shown up yet? We've been waiting for three hours.
Guy 2: Ya know what? FDB, man.
Guy 1: Yeah, fuck dem bitches, let's hit up some other ones.
200👍 140👎