Slang term used to denote the male sex organ, the penis.
One of hundreds of terms, used to denote the male sex organ. Like many such terms, it is based on perceived similarities between the term, and the named organ.
Most are loose adaptations, or humorous takes, on the originating term.
Noteworthy: The motion of the penis, when engaging in sex, can appear to simulate the motion of a jackhammer, in that the shaft moves in quick hard thrusting movements, while the base remains relatively static.
One reason for the term being used in reference to the penis.