We tell someone to "man up!!" when we want them to act more like a man: make decisions, take initiative, get control their feelings and be more macho and tough like Dirty Harry. :P "Are you still moping over your ex-girlfriend? She cheated on you! Man up!!" It works in English by treating the noun like a verb... in this case, a phrasal verb.

it means you as a man you are able to take your own responsibilities.... i've heard this in one of cowboys movies and it means to be strong and though.. hope my explanation is clear :)