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1.
When your pants or jeans hang lower than they normally would, to reveal your undergarments.
that kid tagging over there, is bustin a sag.
by savannaHarlem March 11, 2008
 
2.
to describe someone who is wearing absolutely rediculously baggy pants. the act of wearing said pants creates a sag in the region of the bum of the pants. hence the wearer of said pants is "bustin a sag"
50 cent is bustin a sag in those huge pantaloons
by jordayman November 23, 2005