A demand to be a man and to stop acting craven or cowardly. A derivation of man up
(which in itself is a derivation of cowboy up
), although more insulting. While man up
can be used as encouragement to do something, sack up is almost always a challenge to cease doing something that should be beneath you
"Oh sweet mother of god! The terrorists are going to blow us up with Snapple bottles!" _This_ is terrorism's A-game? Sack up, people.