A term coined by Roy Keane, the Manchester United (football club) Captain at the time. It refers to the 'fans' that go to watch United play at Old Trafford but who actually know nothing about football or United and are only there because United are a very successful team and they want to jump on the social bandwagon of supporting Manchester United.
They will hardly cheer the team on, barely watch the game and be more interested in eating the prawn sandwiches at half-time in the posh player's lounge or wherever (hence the name). They are a minority of United fans, it has to be added, but there are still quite a lot of them.
They will also usually be middle-class and social-climbers, and they are sometimes referred to as the P.S.B
Q. What do you call the glory-supporting, bandwagon-jumping-on socialites that know nothing about football and Manchester United but still go to the games?
A. The Prawn Sandwich Brigade.