An uplifting imperative, derived from the saying "be a man about about it" - to grow a pair, to stop being childish, to stop complaining etc.
This saying can be used in many situations.
1) 'C'mon Gareth, man up! There's no time for tears in this day and age.'
2) After some hurtful comments from a female colleague, Gareth turned to Jimmy for advice. Jimmy said "Grow a pair and man up!"
3) 'Gareth is acting so childish - he needs to man up'