To "be a man about it"; to do the things a man is traditionally expected to do, such as taking responsibility for the consequences of one's actions, displaying bravery or toughness in the face of adversity, providing for one's family, etc. (Compare man.)
Hey son, man up, okay? There will be time for tears later.
(of military personnel) To assemble, each person manning (attending to) his station, prepared for the departure of an aircraft, ship, etc. (Compare man.)
(of other personnel) To staff adequately; to staff up; to successfully fill all needed labor positions. (Compare man.)
To vigorously guard the opponent to which one is assigned.