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.

bumĀ·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.
Breukelen advocaat Reviewed by Breukelen advocaat on . my rant on commonly used phrases the following is a list of commonly used phrases that irk me to no end. so stop using them: 1.'good times'- bad grammar. for the love of god, quit stealing the heading from some myspace page and get some originality. 2. 'bat shit'- often used to describe someone/thing that is crazy. excuse me, bat shit, also known as guano, is very useful. call me bat shit and i might take it as a compliment. 3. 'my bad'- bad grammar strikes again. yes, you are bad. and you're even worse for using such a Rating: 5