To occupy (especially through sexual activity) a hot chick's ugly friend so that your friend can score with the hot one
Hey, Kevin, you mind jumping grenade so I can hook up with the hottie in red?
Acting as a wingman. The phrase comes from the military; jumping on top of an active grenade will kill you but save your comrades around you as a result of it. It's a selfless act for your buddy.
Hey check out those two chicks sitting at the bar. It's a shame only one of them looks decent. Man, will you please jump grenade for me? I did it for you last time.