To egg someone on means to encourage or urge them to do something, often something bold, risky, or mischievous. If your friends egg you on to jump off the high dive when you're nervous, they're cheering and pushing you to go for it. If classmates egg on two students who are arguing, they're making the conflict worse by encouraging it to continue.

The phrase usually suggests the encouragement isn't entirely helpful or wise. When someone eggs you on to do your homework, that's just regular encouragement. But when they egg you on to sneak an extra cookie or talk back to a teacher, they're pushing you toward something that may not be a good idea.

When you egg someone on, you're prodding them into action, much like poking someone to get them moving. The key element is that active pushing: you're supporting and amplifying someone's impulse, urging them forward, for better or worse.