This expression refers to a man's testicle sack, and is used to demand courage and action from another man. In long from it's equivalent to: Grab your sack to make sure your balls are there, be brave and do-it/buy-it/break-up/etc. In shorthand it's: Sack up and do it/buy-it/break-up/etc.
Democrat's need to sack up and stop the reckless, fear-driven war machine that the Neo-Republican party has sadly become.
Republican's need to sack up and admit that they are afraid of almost everything that is not white and Christian, and that their fear is why they want to drop bombs all the time.
Peggy Noonan in reference to Rush Limbaugh and other right wing pundits:
"But I would say of these men (why, in the continuing age of Bill Clinton, does the emoting come from the men?) who are fighting one another as they resist naming the cause for the fight: Sack up, get serious, define."