Really sad British slang that the British use every time they try to define arse.
What's really sad about bum is that if only the British would say ass when they mean ass, instead of mispronouncing arse, they wouldn't need to define arse, and wouldn't feel compelled to say bum.
Someone who wears the same thing to school every single day. Usually a male with a broken backpack that doesn't zip up and only has one arm strap. A Bum must be a fan of the San Francisco 49ers and such a big fan to the point makes people hates the niners. A Bum gets picked on constantly, there are even official "Give a bum a deadleg day" "Beat a bum up day" etc. When a Bum is asked "What can Brown do for you?!" He must reply with "The real question is what can't brown do for you ;]"
Lastly, Not to be confused.
Aliens > Bums
(Weight Training Class)
Bum: (Walks in Late, Ipod headphones in ears, No one is sure if the Bum is sober)
Weight Training Class: "MR BUMMMMMMMMMMMMMMMMM!!!!"