dumb fucking dumby

a step child. Usually one that annoys the piss out of you and needs to have the shit kicked out of them.

Step Dad: I got home last night and the step kid was being annoying as hell
Man: Tell that dumb fucking dumby to shut the fuck up.

Step Dad: You won't believe what the Dumb fucking dumby did last night.
Man: You mean the DFD.
Step Dad: Yeah him