When we say this we refer to men as
rather unhuman beings
machines
as beings who are not supposed to
show their feelings
show their tears
show their hurt
show their passion
show what they love to do
when we say "Man up!"
We usually mean
"dont act like a girl, you are bettter than that"
"dont be this feminine"
"boys dont cry"
"boys shouldnt like ballet"
"boys shouldnt do this, shouldnt do that"
"you run like a girl"

Yes, you can man up.
Man up!
Man up for what you feel is right and not for what society thinks is right.
Man up without bringing yourself down.

Man. Up. For. Your. Self.
and only for yourself.