Some people used "bummer" as short for a bad trip (on something that's not supposed to be mentioned here), "Hey man, it started out good but after a few hours it was a bummer". The word morphed into other usages.

bum·mer (bmr)
n.
1. Slang An adverse reaction to a hallucinogenic drug.
2. Slang One that depresses, frustrates, or disappoints: Getting stranded at the airport was a real bummer.
3.
a. A loafer or idler.
b. A beggar

bummer - definition of bummer by the Free Online Dictionary, Thesaurus and Encyclopedia.