A blowjob from an actual bum that has missing teeth and a mustache/beard combo. Most likely occuring to another bum who has exchanged a few dollars or alcohol/drugs for the service.
Boy 1: oi where's Ricky? he's missed school for weeks.
Boy 2: nah yeah I heard he got a bumjacking by the principal for stealing deano's tuckshop money, apparently Rob from 6A saw him in a wheelchair on the weekend.
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.