The line comes from Fawlty Towers, the BBC sitcom created by John Cleese and Connie Booth, in the 1975 episode The Germans. Basil Fawlty, the hapless hotel owner played by Cleese, warns his staff not to mention the war before German guests arrive, then spends the episode doing exactly that. The warning became one of the show's most quoted moments and a lasting shorthand in British comedy for an awkward subject everyone is trying to avoid. Clipped for soundboards and social video, it still works because the setup is instantly recognisable even without seeing the episode.