"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."
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.