A closed fist hitting of the male genitalia, meant to evoke surprise and/or pain.
Dave Haldane was not prepared for the extreme cockpunching he would receive for not giving Rachel a part in school's production of the Rocky Horror Picture Show.
puncher: hey dude, guess what?
victim: what the fuck do you want, puncher?
puncher: alright I want you to look up real quick
victim: okay
puncher: COCKPUNCH! hahahahaha bitch!