Term meaning to do something that can be very risky. Usually used in a taunting manner, such as demanding that someone "Sack up, bitch." The term is originally derived from the idea that men who take on undue risk can be described as having "big balls." Such large balls would necessitate a big sack to hold them.
Not to cheapen what they did, but the actions of the people on United Airlines Flight 93 on September 11th can be seen as the ultimate case of sacking up.