1. slang Someone prone to behaving in a silly manner. Jimmy's such a goofball—of course he's the class clown. Good luck getting these goofballs to calm down and listen to you!

2. slang Someone deemed stupid and silly. I wouldn't trust that goofball with such an important task.

3. slang A pill with a tranquilizing effect (often because it contains barbiturates), taken recreationally. Tad took a goofball last night and passed out, man.