The movie theater’s most popular concession wasn’t always associated with the movies—in fact, it used to be explicitly banned. Movie theaters were trying to appeal to a highbrow clientele, and didn't want to deal with the distracting trash of concessions—or the distracting noise that snacking during a film would create. f those inside the theaters couldn't see the financial lure of popcorn, enterprising street vendors didn't miss a beat: they bought their own popping machines and sold popcorn outside the theaters to moviegoers before they entered the theater. As Smith explains, early movie theaters literally had signs hung outside their coatrooms, requesting that patrons check their popcorn with their coats. Popcorn, it seems, was the original clandestine movie snack.

Source: Smithsonian Magazine