Someone (usually a BFF) that helps drive away a guy who can't take a hint. A person you've asked in advance to play cock block so you don't have to hurt someone's feelings or pity fuck them.
"Since, Jeff's bringing my phone charger later, I'm watching your kid tonight, Suzanne. He surely won't try to fuck me while I'm holding a newborn. Your baby is going to be the best dick-shield ever."