1. To discourage or make sad. In this usage, a noun or pronoun can be used between "bum" or "out" or after "out." Todd just heard that he didn't get the job, and he's really bummed out. I can't watch the news because it always bums me out.
2. To annoy. My parents were really bummed out when they heard from the principal that I had cut class.
3. To do something badly. I guess I bummed out on the interview because I didn't get the job.
4. To have an unpleasant drug-fueled experience. I don't take that drug anymore because I really bummed out the last time.
Sad or discouraged. Todd just heard that he didn't get the job, and he's really bummed out. I can't watch the news because it always makes me bummed.
See also: bum
mod. discouraged; depressed. I feel so bummed. I think I need a nice hot bath.