-adjective. Feeling really depressed and sad about an issue. gloomy; morose. Affected by unhappiness or grief; sorrowful; mournful. To be in such a saddened state such as when you miss someone or something that you can't stop thinking about that person or something
Dude... my girlfriend couldn't come over today. I REALLY miss her a lot. I feel really bummed out! :(
When you are absolutely positive that you are right, but then end up being proved wrong.
"Ahahah shameeee you just got bummed out!"