butt bus
n. A bus parked near a pub or restaurant that is used as the establishment's smoking section.
Burke rustproofed a former school bus with red paint, put a string of lights on it and added a heater and insulation. His patrons endearingly refer to it as "Puff the Magic Wagon," or the "Butt Bus."
—Paul Burkhardt, “Bar owners in Canadian city find loophole to smoking ban — 'butt buses',” The Associated Press, December 21, 2005
It's happy hour at T. B.'s Pub, but the place is almost empty. The real party is happening just steps away on a smoke-filled red school bus parked beside the bar in a gritty, working-class part of northwest Edmonton.

This is the "butt bus," a place for bar patrons to light up between pints of Molson Canadian and Bud.
—Katherine Harding, “'Butt buses' ignite Edmonton furor,” The Globe and Mail, December 16, 2005
2003 (earliest)
Franco Magnifico figured he had the city's smoking bylaw beat.

If his customers were no longer allowed to smoke inside his hotel bar, he'd provide them with a terrific option. Rather than annoy the neighbours with inebriated riff raff and stray butts everywhere, he'd keep everyone happy. A responsible bar owner worries about these kinds of things.

So Magnifico, owner of the St. Boniface Hotel, played by the rules and put up no-smoking signs inside his infamous Club St. B. Then he spent $500 on an old school bus, parked it in front of his establishment and told patrons they had their own smoking section.
—Lindor Reynolds, “Magnifico butt bus idea busted,” Winnipeg Free Press, November 14, 2003