A hotbox is an air-tight room or vehicle that contains one or more pot-smokers smoking one or more joints. The exhaled smoke and the smoke coming from the joint, unable to escape, circulates and thus is breathed in and is not wasted. Smokers in a hotbox may find themselves totally fucked beyond the point of speech after about 30 minutes. Hotboxing is an event that requires some amount of planning, but ensures optimal weed usage.
Guy 1: "Dude, my parents are on hliday and we're gonna hotbox my bathroom!!"
Guy 2: "SWEEET!"