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1.
-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! :(
by XXXpinoy777 February 16, 2010
 
2.
When you are absolutely positive that you are right, but then end up being proved wrong.
"Ahahah shameeee you just got bummed out!"
by Anjileen September 24, 2007