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Noun. The gentle but devastating blow that hits your central nervous system courtesy of certain opiates. The pillowhammer lays you out and you stay down, because it's pretty relaxing down there.
A: "Hey did you ever notice how ...."

B: "Planning on finishing that sentence?"

A: "...Huh? Oh, sorry, pillowhammer just kicked in."

An description of the pillowhammer, taken from an article by Angie Chuang about propofol use :

"However, taking propofol was one of the most truly pleasant pharmaceutical experiences. Propofol didn’t just make me sleepy; it made me feel as if my soul had been detached from my body and was resting just above it, on a cloud of marshmallow fluff, sweet and achingly tender. It made the time I was under its influence pass in a flicker like the inside of a tunnel viewed from a bullet train, just a glimpse of darkness and lights stretched into long streaks."
by DrMexico September 16, 2009