The unused side of one's full/queen/king sized bed that when not being used by someone else is used as a back up for when you come home from a long day of work and realized you have no time or you're just too lazy to take a shower that night, preserving the freshness of one's normal side of the bed.
John: Holy crap dude yesterday was a kick in the nuts, I didn't get home till 3 in the morning.
Clyde: Damn man you atleast took a shower right?
John: Nah I waited till this morning, I slept on the other side of the bed, I hate when I fall asleep and my sheets feel hot and sticky as hell.
Clyde: I hear ya man