(idiomatic) to "be a man about it"; to do the things a good 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.
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.
(idiomatic) (of military personnel in a unit) to assemble, each person manning (attending to) his station, prepared for departure of an aircraft, ship, etc.
(idiomatic) (of personnel, industrial, etc.) to staff adequately; to staff up; to successfully fill all needed labor positions.
(sports) to vigorously guard the opponent to which one is assigned
2005 Allan Trimble, Coaching football successfully, Human Kinetics, p166
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,p138
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, p141
I had to "man up" on their rover, and every time I went for the ball he tried to hit me.