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1.
"Showing some sack" is used to describe an action, behavior or display of ostentatious swagger -- an act that is perceived as extremely bold, audacious, or risky.

Similar to having "cajones."
Stephen Colbert was seriously showing some sack when he derided and embarrassed George W. Bush at the 2006 White House Correspondents Association Dinner.

It was quite possibly one of the finest moments in comedic history, I might add.
by MaxwellX August 29, 2010