1. To sadden one or cause one to feel disappointed. In this usage, a noun or pronoun can be used between "bum" and "out." I can't watch the news because it always bums me out. Todd just heard that he didn't get the job, and he's really bummed 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 experience with a recreational drug. I don't take that drug anymore because I really bummed out the last time.