The line comes from The Princess Bride, Rob Reiner's 1987 fantasy adventure film, spoken by Miracle Max, the folksy miracle worker played by Billy Crystal. He delivers it as Westley, Buttercup and Inigo set out to storm Prince Humperdinck's castle, a mission everyone treats as near-suicidal. The film grew into a cult classic on home video, and the quote became one of its most repeated lines. Online, it circulates as a short reaction clip, a wry good-luck wish for plans that are obviously doomed.