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.
by Quine August 13, 2006

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Top Definition
To gather up one's courage for a daunting task.
Sargeant Bohannon shrieked at Private Entwhistle, "Sack up, you panty-wearing dipstick...bayonet practice may save your life...now lunge, thrust, stick and twist...repeat...
by harry flashman July 28, 2003

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Stop being such a pussy
You need to sack up instead of crying about your pussy hurting. (see also wah my pussy hurts.
by jaydawg May 27, 2004

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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."
by Marko D February 08, 2008

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The figurative act of growing a scrotum and pull of a crazy stunt or to face some sort of tough task
Sack up u pussy and fuck dat bitch
by L-White April 16, 2008

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