Smooth, as cool as the other side of the pillow, the shit or simply something that is top notch or untouchable.
Could be used by saying "Stay Durden" for signing an email or instead of ending a phone call with "bye." It means to stay chill or cool. It could also be used in saying that shit is "Tyler Durden" which means that you can't get any cooler than that.
by Mark Ledge September 13, 2005