Dumbcrap is a word generally used as an insult to another person which is formed from the two words: Dumb and Crap. You are essentially calling someone Dumb and a 'Crap'.
When you get stoned to the point where you're glued to the couch, and can't can't make any truly meaningful contributions to conversation; the term "dumb" refers to both the person's current level of retardation as well as their relative inability to speak.
"Yo I'm bout to go get dumbclapped and watch Rick and Morty"