I assume this came from those 80's games in like in tudor where you had to fight 'a boss' at the end of the level.
It was a catch phase from the 70s TV show Charlie's angels.
Often after beating a baddie or winning something they'd collectively remark - like a boss - which was an acronym for Brilliant Old Success Story.
Good stuff norrie.
Another little known fact is that Mick lyons and Ballsaq both signed up for the PROC on 24 dec 1976.