In The Princess Bride, Vizzini repeatedly uses the word “Inconceivable!” to express disbelief.
Vizzini says the word whenever events contradict his confidence, including moments involving the Man in Black and the pursuit across the sea. His repeated exclamation becomes a running joke because his certainty is repeatedly undermined by what he sees.
Rob Reiner directed the 1987 film, adapted from William Goldman's 1973 novel. Wallace Shawn played Vizzini, a Sicilian criminal mastermind who travels with Fezzik and Inigo Montoya. The screenplay preserves the character's theatrical verbal style.
The word is sometimes attributed to another character because it recurs so prominently in discussions of the film. In the movie, however, “Inconceivable!” is Vizzini's signature expression. The humor depends on the gap between his self-image as an infallible strategist and his frequent misreadings.