Clap box?

The phrase comes from the battacchio—called the 'X' in English—a club-like object composed of two wooden slats used in Commedia dell'arte. When struck, the battacchio produces a loud smacking noise, though little force is transferred from the object to the person being struck. Actors may thus hit one another repeatedly with great audible effect while causing very little actual physical damage. Along with the inflatable bladder (of which the whoopee cushion is a modern variant), it was among the earliest forms of special effects that could be carried on one's person.