In extreme circumstances, it could also describe an ugly experience with drugs, violence or sex.
Bummer is a shortened version of bum scene.
The word 'bum' would often be said while holding in a hit of weed followed by the word 'scene' that would be stretched out while exhaling the hit. As in: Bum (holding breath) sceeeeeene (exhaling slowly).
#2: Bum scene, man.
#1: I was so messed up I think I left it at your girlfriend's house.
#2: Bum (holding breath) sceeeeeene (exhaling slowly).