It means if your boyfriend or girlfriend has cheated on you, you should do the same.
If bf/gf has given you "bun" or you are interested in giving gf/bf "bun" it simply means to cheat on them.
*the right definition*
Girl 2: My girl nuh cry bout dat but nuh "bun him" bac jus gi him di bakery and dun
- simply meaning don't just cheat on him might as well get pregnant while your at it =D
Can be used in any situation to express dislike in a person, place or thing.
Derived from the patois word bun which means to literally light something on fire.
Gyal 2: BUN HIM!!!!!!!
Mike: Man.. my mom won't let me go to the jam tonight..
John: Ah fuck that, bun her!!