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1.
When you are fucking a girl doggie-style and her back is hunched over.
Steve: Hey, I heard you fucked Diane last night.

Ted: Yea I was hitting it doggie but she had man-back and it really turned me off.

Steve: That sucks, my girl arches it perfectly.
by murf09 July 01, 2009
 
2.
When a woman has a very masculine back. This disturbing and very unfeminine feature includes any or all of these features: broad shoulders, large back muscles (lats and traps), or lots of moles or freckles.
Girlfriend: "Is that a man or a woman?"
Guy: "It's a girl, but that's some serious man back!"
by Gitta December 23, 2005