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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.
In New Zealand, "bummed out" can have a different meaning from just being depressed or sad.

It can also be commonly used here to describe someone who thought they were so right and very insistent about how correct they were about something, but then ends up being proved wrong.
When this happens you can say that person has been "bummed out".

A similar meaning and often preceding saying to "bummed out" is the interjection, "shame!". Indicating that the person should feel shame (or even "shamed out") for being wrong when they thought they were so right.
Person 1 - Hey, did you know that pineapples don't grow on trees?
Person 2 - Yes they do, don't be so dumb, you idiot!
Person 1 - Go google it then...
Person 2 - Oh yea, true, guess you're right.
Person 1 - Oh shame! You just got bummed out bro!
by Knowa August 07, 2014
 
3.
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