To nag means to complain or make requests repeatedly in an annoying way. When someone nags, they keep bringing up the same thing over and over: “Did you clean your room yet? What about now? Why haven't you done it?” A nagger might remind you about the same chore five times in an hour, even after you've said you'll do it.

Parents sometimes nag their kids about homework or chores, but kids can nag parents too, pestering them with “Are we there yet?” every few minutes on a long car ride. The key element of nagging is the repetition. Asking once is a request; asking ten times is nagging.

People usually nag because they're worried something won't get done, but nagging often backfires. Instead of motivating people, it frustrates them and makes them less cooperative. When you feel nagged, you might dig in your heels and resist doing the very thing being requested.

As an adjective, nagging can also describe a persistent, annoying feeling, like a nagging worry that won't go away or a nagging pain in your knee that keeps bothering you throughout the day.