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Masturbating by putting your penis between the mattress and the box-spring.
"Oh like when you put it between the mattress and the box-spring? That's called the lonely sandwich" - You Look Nice Today: The Magic Number -
by terriakki chan July 12, 2010
The act of cutting the zipper off of a plastic sandwich bag, squirting lubricant into it, placing the plastic bag into the microwave for 4-6 seconds, placing the bag between the cushions of a couch, and proceeding to then fuck the couch silly. Usually done by lonely virgins.
Guy 1: Dude, I lonely sandwiched like three times last night.

Guy 2: You're a sick bastard. You seriously need to get laid.
by otacon33 July 13, 2010