Pessimism means thinking the worst. It's the opposite of optimism, which means assuming the best. Most people feel that too much pessimism is paralyzing — who will every try anything if they are always sure they will fail?
Pessimism comes in two flavors. With one, you feel that something bad will happen. You feel pessimistic about the geometry final. How will you pass it? Pessimism is also the tendency to expect the worst in everything. A pessimist in love is a person sure that there must be something wrong with the person to love them back. A pessimist on an airplane is sure the plane is missing some essential part that won't be noticed until it plummets from the sky. Can pessimism be fun? Not really ever.