From the Super Mario Wiki
The Monster's Hammer is a battering item, much like the Thunder Stick. Upon using the Monster's Hammer, the user will proceed to swing it around for about 5 seconds, dealing damage to any other racers who draw near and causing them to lose speed.
A variant of the Monster's Hammer known as the Monster's Hammer+ exists exclusively in Mario Kart Arcade GP. This variant extends the duration of the attack by around 2 seconds.