The line comes from the 1980 comedy film Airplane, where Leslie Nielsen plays Dr Rumack. In the scene, Ted Striker says surely you cannot be serious, and Rumack replies that he is serious and not to call him Shirley, turning a tense cockpit exchange into wordplay on the name Shirley and the word surely. The joke became one of the film's most quoted moments and later spread through television reruns, comedy compilations and online meme videos, where the deadpan delivery still works as a quick, instantly recognisable punchline for disbelief.