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1.
A promiscuous woman who engages in coitus with many different partners and thus should attach a mattress to her back to increase time efficiency.
Ay Wortimer! Do you recall that matt back we double-teamed last Friday?
by Nick Ryan and Dean Gomez September 01, 2007
 
2.
A large fatty growth that makes your back and head look like they are one solid piece. Often it makes you walk like a penguin.
Move, your Mattback is covering the sky.
by Jebuscriph March 12, 2008