Green’s Dictionary of Slang

1. (US, also whaddarya?) a rhetorical question, used more as an affirmation than a query.

1951		Cusack & James Come in Spinner (1960) 320: ‘Well whaddayaknow!’ Sport chuckled. ‘Is that a fact?’.
1988		McGill Dict. of Kiwi Sl. 122/2: whaddarya? rhetorically questioned contempt.
1999		Observer Rev. 3 Oct. 5: I bet it anyway and whaddya know, the Hat has nothing but a lousy king-10.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. see what do you know? phr.