Bummer (noun)

1. A disappointment or a letdown.
2. An unpleasant or disappointing experience or situation.

Origin:

Slang term, origin not fully known but it may have originated from the word "bum" which means a tramp or a homeless person.

Examples:

1. It was a real bummer when the concert got rained out.
2. It's a bummer that I have to work on my birthday.
3. The party was a bummer, nobody showed up.
4. It was a bummer to find out that the concert was sold out.
5. It was a real bummer when he didn't get the job he applied for.