A pest is something that bothers, annoys, or causes problems, especially when it keeps happening and won't go away. The word usually refers to insects or small animals that damage crops, invade homes, or spread disease. Mosquitoes are pests because they bite and spread illnesses. Rats become pests when they get into grain stores and contaminate food. Locusts can swarm across farmland in massive clouds, eating entire fields of wheat in minutes.

But pest also describes a persistently annoying person, especially someone who won't leave you alone when you're trying to concentrate. If your little brother keeps interrupting while you're reading, poking you and asking “whatcha doing?” every thirty seconds, he's being a pest. The key is repetition: someone who bothers you once isn't a pest, but someone who bothers you over and over again definitely is.

Farmers spend enormous effort fighting agricultural pests, using everything from scarecrows to pesticides to keep insects and rodents from destroying their harvests. When people call someone a pest, though, they usually mean it more playfully than seriously, like when a teacher smiles and says, “You kids are such pests today,” because the class won't settle down.