Wiktionary
paraprosdokian
n. figure of speech in which the latter part of a sentence or phrase is surprising or unexpected; frequently used in a humorous situation.
Wikipedia
Paraprosdokian
A paraprosdokian is a figure of speech in which the latter part of a sentence, phrase, or larger discourse is surprising or unexpected in a way that causes the reader or listener to reframe or reinterpret the first part. It is frequently used for humorous or dramatic effect, sometimes producing an anticlimax. For this reason, it is extremely popular among comedians and satirists. Some paraprosdokians not only change the meaning of an early phrase, but they also play on the double meaning of a particular word, creating a form of syllepsis.