When someone says a gay joke, something stupid, or something that deserves to be punished for. You also say beefneck while slapping the back part of their neck. Beefneck is a way to say that something is dumb; some people confess and put forward their necks and get hit, voluntarily.
Jimmy: Basketball is one of the dumbest sports ever.
(Slaps back of neck)
reaction to when someone says something stupid, you slap them on the back of the neck while saying "beefneck"
Person #1: "Megan Fox isn't hot"
Person #2: "Beefneck"