Cowboy Up is something that the deer urine bat soaker Kevin Millar came up with. It basically means that they are grinders. If you are hurt, not feeling well, you have to "Cowboy Up". All in all, he is comparing the team to a bunch of cowboys. Personally, I find it pretty lame and the homoerotic overtones are astonishing.
Anyway, I think the Sox pull this one out but go on to lose the series in six games. The Cubs should win as well since I think Matt Clement is going to do well against his old team.