originally used in context to video games in which, if a player's character was on the verge of death, one would shout to the player to "man-up." has elvoved into meaning "stop being a big wussy baby."
Dude, stop crying and man-up! (or)
Tony: Dude, can we take a break from the Wii? My hands are cramping?
Ralph: Man-up, wuss!