When faced with a hard chore, it's a shift in attitude from "can't" to a positive "can-do" with confidence and a non-complaining spirit that becomes contageous.
Halfway through a hot day of hauling hay you might tell yourself or your buddy to "c'mon, cow-boy up." and thereby keep your work and your attitude from going south.
Basically, another way of saying "Shut up and take it like a man" or "Quit your whining." The term is derived from the popular image of cowboys being tough, unflinching, uncomplaining, and hard-working.
Essentiallyanother term for self-endowment of strength and moral, or to another. Essentially another term for a similar phrase like, "toughen up", or "stop your belly-aching and fight!", etc etc.
The original meaning of "Cowboy Up" was to inform the next bullrider or bucking bronco rider at a rodeo to get ready to ride or "cowboy up". Now it means to stop being a pussy liberal snowflake with your safe spaces and whiny bullshit, grow up and be a man for once in your pathetic life.
Liberal snowflake type: Oh no! Trump won the election! I need to find a safe space to calm down and stop crying.