(idiomatic) 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 (“brace oneself, steel oneself”).)
I was wondering when he would man up and marry that girl he knocked up.
You need to man up and confront your boss about his behavior.
Hey son, man up, ok? 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 (“supply with staff; take up position to operate”).)
(of other personnel) To staff adequately; to staff up; to successfully fill all needed labor positions. (Compare man (“supply with staff”).)
(sports) To vigorously guard the opponent to which one is assigned.
2005 Allan Trimble, Coaching football successfully, Human Kinetics, page 166:
They must be athletic enough to man up on a solid number two receiver and yet be physical enough to step into the line scrimmage and tackle a running back.
2005 Andrew McLeod & Trevor D. Jaques, Australian football: steps to success,page 138:
He will man up on his opponent whenever his opponent has the ball.
2007 Blye Frank & Kevin G. Davison, Masculinities and schooling: international practices and perspectives, Althouse Press, page 141:
I had to "man up" on their rover, and every time I went for the ball he tried to hit me.