fun (fn) n. 1. A source of enjoyment, amusement, or pleasure. 2. Enjoyment; amusement: have fun at the beach. 3. Playful, often noisy, activity. intr.v. funned, fun·ning, funs Informal To behave playfully; joke. adj. Informal Enjoyable; amusing: "You're a real fun guy" (Margaret Truman). Idiom: for/in fun As a joke; playfully.