The act of pretending that the knuckle of your hand is a stubby penis. This act is performed to shock onlookers.
One performs this act by covering all but the knuckle of one hand with the other hand, and placing it over one's genitals.
Knuckle Penis is usually performed over clothes, but it is not unheard of to perform the act underneath clothes, and have only the knuckle exposed through the fly.
"On no, Pops is doing knuckle penis again!?"