A "party pooper" or a "killjoy" is someone who spoils the enjoyment or enthusiasm of others. They might do this by being negative, overly critical, or refusing to participate in the fun. For example, at a lively birthday party, if someone keeps complaining about the noise or the food, that person is being a killjoy. They dampen the positive mood and make others feel less excited or happy. People generally don't like being around a killjoy as they bring down the energy and can turn a pleasant gathering into a dull or uncomfortable situation.
