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1.
Closely linked to the word "post material", smash stack refers to two or more hot girls in the same areah or room.

Smash meaning having sex with a chick and stack meaning more than one
Guy #1- You see that post material in the corner?
Guy #2- Yeah n she bought her friends too, complete SMASH STACK!
by IST Slang 2012 September 02, 2009