A reference to the male ass. Since a female vagina has a vertical slit, much like the ass, it is easy to see where the term derives it's meaning. It can also refer to the anus of a man, though this would be erroneous as the anus looks nothing like a vagina unless both have been dilated, in which case both are gaping and disgusting.
"Man, my back pussy is sore from all those squats."
by The Masticator Part Deux January 27, 2012