What do you know?

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(well) what do you know

A phrase used to indicate one's surprise. Well, what do you know—we have chocolate chips in the cabinet after all.
See also: know, what

What does (one) know?

A rhetorical question meant to indicate that one's opinion or claim isn't trustworthy, believable, or welcome. Sometimes used ironically or sarcastically. A: "You really shouldn't be eating so much red meat." B: "Ah, what do you know? Red meat helps make you strong!" A: "All the healthcare experts out there say babies ought to be breastfed exclusively." B: "Bah, what do they know? My siblings and I were all bottle fed, and we turned out just fine." So, what, you think I'm wrong about climate change because of some article you read on Facebook? I mean, I only have a PhD in climatology, what do I know?"
See also: does, what

What do you know (about that)?

Inf. That is very interesting. Tom: I heard that Jim and Mary are getting married. Jane: Well! What do you know about that? What do you know? Bill finally sold his house!
See also: what

What do you know?

Inf. a typical inquiry on greeting someone. (A specific answer is not expected. Often pronounced "Wha-da-ya know?") Bob: Hey, Tom! What do you know? Tom: Look who's here! Hi, Bob! John: What do you know? Mary: Nothing. How are you? John: Okay.
See also: what

what do you know (about that)?

used as an expression of surprise. informal, chiefly North American
See also: what

(well) what do you ˈknow (about ˈthat)?

(informal) used to express surprise: Well, what do you know? Look who’s here!
See also: what