A guy friend that a girl will use to protect her from being hit on. It is preferable though not necessary for this guy to be gay. If a girl is not interested in a guy who approaches her she will suggest her cock shield friend is her boyfriend. If an interesting guy makes an advance on her she will assure him that the cock shield is merely a friend. An effective decoy tactic deployed by women in order to prevent the awkward advances of undesirable men.
"Dude what are you doing tonight? Wanna come chill with me?"
"Nah, sorry man I promised Kim I'd be her Cock Shield tonight"
2. A defensive and honorable maneuver where a friend successfully has sex with a chick that is stalking you, freeing you up to score with someone else. The need for the cock shield arises when the act of having sex with the stalker will lead to a bunch of bad consequences.
This is similar but subtly different to being a wing man, a cock block and the cock shield def 1. as your friend is trying to avoid you having sex with one person so that you may have sex with another.
Dude, thanks for the cock shield last night. I now have at least another month or so before she'll be back in full force tying to get me to sleep with her again - nightmare averted!