A blackjack is a deadly weapon.
It consists of a thick leather tube, less than a foot in length, with a cylindrical lead slug in each end and a stiff spring in the middle. In a typical blackjack, the diameter of the slugs is greater than the diameter of the spring. In the past, before metal springs were available, multiple flexible slivers of springy wood were used to create the spring action.
To use it, grip one end and smack the other end against an opponent. The force that a blackjack generates will crush bones. A strike to the head will likely result in a killing blow.
A sap is sometimes confused with a blackjack, however it is not a deadly weapon. It's a narrow leather bag filled with fine lead shot that is gripped in the fist and extends out to one side where it can add a considerable amount of extra force to a punch. The loose nature of the shot prevents it from shattering bone.