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.