act the fool

1. To behave comically or playfully, often to amuse others. If you continue to act the fool, you're going to have to leave the classroom. Joe never acts the fool—he's always serious. Why do you have detention? Please tell me you weren't acting the fool again.

2. To behave irresponsibly or immaturely, especially in an environment where it is especially inappropriate. Don't let me catch any of you kids acting the fool at the museum—it's a privilege to even go inside. We finally get invited to a nice dinner party and you have to go and act the fool—what's wrong with you? People who've had too much to drink always want to go down to the pier and act the fool, hollering and carrying on.

act/play the ˈfool

behave in a stupid way to make people laugh, especially in a way that may also annoy them: It’s impossible to have a decent game of tennis with Frank — he acts the fool the whole time. ♢ If you played the fool in class a little less and worked a bit harder, you could do quite well.

In the past, a fool was a man employed by a king or queen to entertain people by telling jokes, singing songs, etc.