The line comes from Monty Python and the Holy Grail, released in 1975, during the French castle sequence where the knights are taunted from the walls instead of being allowed inside. After the guards shout for the cow, it is catapulted over the battlements in one of the film's most quoted visual gags. Clips of the scene circulated on video sites and soundboards for years, and the phrase survived because it is short, instantly recognisable to comedy fans, and works even without the picture as a signal that something ridiculous is about to happen.