That reaction dates back to Gibbs junior high school days, because "man up!" was what he heard right before being stuffed in his locker -- or getting creamed (again!) with the ol' dodgeball...
Junior High was a long, painful six years for ol' Gibbsy and he's never quite gotten over it.
Look at Gibbs’ education and then see why he NEVER “maned up” : http://en.wikipedia.org/wiki/Robert_Gibbs