The act of hotboxing a car with marijuana. This sublime effect is achieved by sealing all the doors and windows and smoking copiously, preferably while driving around aimlessly (so as to avoid detection). Do not, under any circumstances, pick up little girls while you are stoned, no matter how much you are tempted to. This is in reference to the film "My Neighbor Totoro," in which the catbus is a whimsical vehicle used by the main characters.
"We totally catbused Isaac's car last night."
"His upholstry stinks of weed ever since our last round of catbusing."
"Hey man, I just bought an eighth and my roommate gave me his car keys. You wanna catbus?"
"Hey hey hey, ride the catbus every day." (this last one is an apocryphal quote from Dr. Dre)