Derived from the football field. Players were taught to keep their head up or "bow their necks" so that when they tackle someone they didn't break their neck.
Socially it means:

1) Tell someone to stop being a vagina and accomplish whatever task of varying difficulty is at hand.
2) Tell your buddy to bow the neck of their cock (get a hard on) and engage in sexual intercourse with a girl.

A casual invitation / threat used by an individual that is not afraid of fucking someone up.