similar to a hotbox, a slopbox is any vehicle inside of which alcoholic beverages are imbibed. Said vehicle is usually stationary and parked in a residential area, grocery store parking lot, or a "slopbox drive" (a secluded road infamous for frequent slopboxing practices). It is common and accepted to exceed the vehicle's maxiumum capacity whilst slopboxing.
E: We've gotta get drunk off P's liquor, but where can we do it?
K: Not my house, my parents are home!
G: I know, we can slopbox E's car in town center!