Bummer has many definitions, most of which I’m sure you’re aware.
But we’re not bothered with the conventional meanings are we?
A bummer, and this is a definition peculiar to the English Midlands – unless someone proves me wrong – is somebody who performs anal sex, either heterosexually or homosexually; the giver rather than the receiver.
Obviously, it’s quite derogatory in the latter case. The purpose of this post is not to educate you in derogatory slang and colloquiallisms but rather to make you aware of them, should you ever travel.
It comes from the British slang noun, Bum, which means ‘The Buttocks.’
Being called a name and not knowing what it means can lead to all kinds of trouble. Especially if you think it’s a nice word when it’s not friendly at all, in which cases a trip to the fractures clinic may be in order.