Paraprosdokian: figure of speech in which the end of a sentence or phrase takes an unexpected twist. From Greek para (against or beyond) and prosdokia (expectation), meaning “contrary to expectation.”

Examples:

• Groucho Marx: “I’ve had a perfectly wonderful evening, but this wasn’t it.”
• Winston Churchill: “You can always count on the Americans to do the right thing—after they’ve tried everything else.”