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When walking up the stairs at school and the person in front of you has a huge backpack and it smacks you in the face. Another way you can endure pack smack is when someone turns suddenly in the hallway, their backpack hits you, thus pushing you over.
Joni just pack smacked me right into those lockers.

Man, That Josh kid totally pack smacked me in the face today.
by plasticbeach November 02, 2010