When you fail to meet people's expectations, you disappoint them. You can also disappoint yourself or find that things let you down, like a rainy trip to the beach that disappoints you.
The word disappoint comes from the Middle French word desappointer, which meant "dispossess of appointed office," in the 14th century. Today, disappoint can apply to anyone or anything that does not achieve the expected results — and ways to express that we are disappointed go beyond firing people or kicking them out of office. For example, if you buy a product that disappoints, you won't buy it again.