The act of connecting your fist, at high velocity, with the genitals of a member of the male sex.
It's generally considered that the true cock punch is delivered from a squating posistion, and so is delivered as a jab rather than a uppercut motion.
Protaganist #1 - I'm gonna shag your mum
Protaganist #2 - not tonight your not
Protaganist #1 - AAAAAHHHHHHHH!
protaganist #2 - no one expects the cock punch