Verb used to describe the act of "throwingsomeone under the bus" or otherwise admitting someone's wrongdoing for them.
My sister was in trouble with my mom, so she underbussed me for something I did last week, and got herself off the hook.
An underbusser is a grown tattletale.
When you have just taken to many drugs over the weekend, its sunday night work starts tomorrow and you are paronoid and depressed the underbus picks you up.