Bumn is the swear version of bum (as in, ass). Just as damn is the swear version of dam. (Well, not really a 'swear,' but certainly not something you'd hear a child say!)
Unlike damn and dam, however, bumn DOES have the same meaning as bum.
Person A: "Goddammit! I have shit stains on my underwear again!"
Person B: "Well, if you'd just wipe your damn bumn better, you wouldn't have that problem, moron, now would you?"