A Chumscrubber is someone who helps clean up a friend after they've made a big mess of themselves. They save the day by scrubbing up the mess.
1: John Bradford got trashed last night, he vommed all over himself and shit his pants.
2: Mate, what did you do?
1: I was the chumscrubber, i cleaned that boy up like the amazing friend i am.
2: Your the best mate.