Slept has 2 meanings
To get high
Slept is a term that describes a person getting high from marijuana. It comes from the similarity between a person sleeping and the comatose state of being under the influence of weed.
One of the side effects of taking a hit is increased drowsiness. The active cannabinoids in marijuana copies the effects of a person's natural sleep regulators and starts the body's natural sleep/wake cycle.
You got the good stuff? I need to get slept tonite.
Slept is a term that refers to when you hit someone so hard you knock him out. It comes from the similarity between someone sleeping and being knocked unconscious.
Slept is often used gleefully to gloat about how one person physically destroyed another person. It is most commonly seen and heard in boxing and UFC matches.