A louse is a tiny wingless insect that lives as a parasite on the bodies of humans and other animals, feeding on blood or skin. These pests are about the size of a sesame seed and can cause intense itching. Head lice are the most common type that affects children, spreading easily when kids share hats, brushes, or lean their heads together. Finding lice doesn't mean someone is dirty: these insects can infest anyone's hair, regardless of how clean they are.

The plural of louse is lice (you wouldn't say “louses”). Schools sometimes send home notices when lice are spreading, and parents check their children's hair carefully, looking for the tiny insects or their eggs, called nits.

The word louse can also describe a person who behaves in a contemptible or mean-spirited way. If someone betrays a friend or acts selfishly without caring who gets hurt, you might hear them called a louse. This usage is old-fashioned and pretty harsh, comparing the person to an annoying parasite. You might encounter it in older books or movies where a character says something like “He's a real louse for cheating a business partner.”