This phrase is an adjective used to describe someone who's demeanor is so cool that if you don't like them, there is something wrong with you; a literal description based on waking up at night, flipping your pillow, and the coolness making you feel so good you fall back to sleep.
Chris? Oh that dude is the shit! He's cooler than the other side of the pillow.
by Cunninglinguist69 July 31, 2006