(colloquial, set phrase, rhetorical question, often sarcastic) What a surprise; guess what?
Well, what do you know, it's raining again.
Used other than figuratively or idiomatically: see what, do, you, know.
Source: en.wiktionary.orgI was seven years old. What do you know when you're seven years old? All my life, or so I thought, we'd been in the city of Alexandria, in the Street of the Carpenters, with the other Galileans, and sooner or later we were going home. Anne Rice
Write what you know, and what do you know better than your own secrets? Raymond Carver
Who are you, gaijin? What do you know about honor?' 'I'm called Chocho,' Will said... 'Chocho?' Arisaka shouted, goaded beyond control. 'Butterfly? Then die, Butterfly! John Flanagan
What do you know about the activities of the brain and the nervous system? George Alec Effinger
We are as forlorn as children lost in the woods. When you stand in front of me and look at me, what do you know of the griefs that are in me and what do I know of yours? Franz Kafka
What do you know about dragons?” "They're big, scaly, four-legged creatures with wings who terrorized small villages until a virgin was offered up as a sacrifice.” His grinned again. "I do miss the virgins. Katie MacAlister