joke

If you tell a joke, you say something to make people laugh.

Example: Trevor's joke about the boring lexicographer was too funny.

If something is a joke, it is not considered seriously.

Example: Don't listen to Joseph. He's a joke. Nobody pays any attention to him.

When you joke about something, you say something funny that you don't really mean.

Example: Everyone joked about how bad his tennis was.

You say "you must be joking" when somebody tells you something that's very hard to believe.

Example: "She's forty-three." "Forty-three?! You must be joking. She looks like she's sixteen."