Informal:
* Bummed: This is a common slang word for feeling sad or disappointed.
* Down: Similar to bummed, but can also mean depressed or lacking energy.
* Blue: A classic slang word for feeling sad or melancholic.
* Dejected: Means feeling discouraged or disheartened.
* Gutted: Indicates feeling deeply disappointed or upset.
* Down in the dumps: This is a more old-fashioned expression for being sad or unhappy.
* Depressed: While not technically slang, it is often used informally to describe feeling sad or low.
* Miserable: This word suggests feeling extremely unhappy or uncomfortable.
More informal/juvenile:
* Crip: A slang word for feeling sad or depressed, often used in a humorous way.
* Sucky: Used to express disappointment or sadness about something.
* Crappy: Similar to sucky, but more informal.
Remember that the appropriate slang word will depend on the situation and the audience.