Pessimistic describes the state of mind of someone who always expects the worst. A pessimistic attitude isn't very hopeful, shows little optimism, and can be a downer for everyone else.
To be pessimistic means you believe evil outweighs the good and that bad things are more likely to happen. So pessimistic people are usually pretty negative. And kind of a bummer to be around. Think of Eeyore, Winnie the Pooh's perpetually gloomy friend. The word comes to us, not surprisingly, from the Latin pessimus — meaning "worst."