A question that precedes a surprising announcement. It is a set phrase and is often but not always rhetorical. Guess what? Your mother is coming to visit this weekend after all. A: "Guess what?" B: "I have no idea—do you have news?"
Inf. a way of starting a conversation; a way of forcing someone into a conversation. Alice: Guess what! Bob: I don't know. What? Alice: I'm going to Europe this summer. Bob: That's very nice. John: Guess what! Jane: What? John: Mary is going to have a baby. Jane: Oh, that's great!
See also: guess