From military wiki:
The slapjack (also commonly known as a "slap"😉 is a batting weapon which may be used in self-defense or other forms of hand-to-hand combat. It typically consists of a flat profile outer skin containing a hard weight material that may be swung at any part of an opponent's body in order to inflict injury. The outer sack of a slapjack may be made of various cloth materials such as a sock; however, higher quality slapjacks are most often made of a stronger material such as leather. This usually features a pouch for the inner "club" and an extension for a handle. Various objects or materials may be used for the inner "club" as well, but the most favored material is lead, for its heavy weight and dense consistency.
Especially in the 1940s, the slapjack has often been associated with street gangs and thugs as the weapon of choice. This was commonly portrayed in American comic books of the era. In more recent times, some professional wrestling heels have also become known for using the slapjack as their signature weapon, namely Konnan and Stevie Ray.