To remove someone from contention "biff them out" or "biff someone out" was a slang term. Originally coined in the Midwestern U.S., in the 1960's common usage, the phrase was used to describe removing someone from contention, preventing them from succeeding, as in "they were biffed out" or eliminated from competition. In sports, it could mean being wiped out of competition, or to fail.
Biff them out. 1. They were biffed out of any chance at success when their past criminal history was exposed. 2. She was biffed out of any chance of winning.
"Mon" is "come on" in Scottish. "The Biff" is what fans call the Scottish rock band "Biffy Clyro". Mon the biff therefor means something like "Come on Biffy Clyro". Fans scream this at concerts just before the show and in between songs.