Noun: A derogatory term used to describe someone who is so retarded that they were likely conceived as an atrociously tragic accident through penetrative bumsex.
Bill: Did you see Ali on Friday night acting like a complete donkey?
Ted: Yes! She's such a fucking bumbaby.
Bill: Dude. Sweet call.
A ButtBabyy is any child that results from semen seeping into the vagina after anal sex. ButtBaby's can be prevented by using a condom, birth control pills, other contraceptives, and by simply not engaging in anal sex.
Father:"Son, now that your 16, its time we tell you that you werent planned, you were a ButtBaby."