Green’s Dictionary of Slang

1. an excl. of surprise, usu. ironic.

1900		Sporting Times 6 Jan. 1/5: Bill, what do you think — my wife has taken her hook with Curly Tom.
1917		Wodehouse ‘At Geisenheimer’s’ in Man with Two Left Feet 116: But, well, what do you know about that?
1947		C. Willingham End as a Man (1952) 108: ‘Well, what do you know?’ said Wilson softly.
1956		I. Fleming Diamonds Are Forever (1958) 160: Whaddya know, Booful! It’s twenty Grand for a rub these days, Boy-oh-boy!
1962		T. Berger Reinhart in Love (1963) 180: ‘What do you know,’ the driver muttered [...] ‘Much obliged, fellow.’.
1980		(con. 1940s) O. Manning Sum of Things 433: Well, what d’ya know! [...] She’s not Persian after all.
1988		McGill Dict. of Kiwi Sl. 121/2: well, what do you know? mild, rhetorical, mock incredulity.
2003		McGill Reed Dict. of N.Z. Sl. [as cit. 1988].

2. a greeting, ‘hello and how are you?’, ‘what are you/have you been doing?’.

1951		Phila. Eve. Bulletin 11 Nov. n.p.: Remember way back then you used to say ‘whatcha know, Joe?’ ’Tain’t like that no more, kid. Now you say ‘Let’s talk trash’ or else ask ‘What’s shakin’?’ The answer is: ‘Nothin’ but the bacon.’.
1965		W. Dick Bunch of Ratbags 226: Hallo, Joe, whaddyah know?
1996		(con. 1946) G. Pelecanos Big Blowdown (1999) 45: Hey, Pete! What d’ya know?
1999		Observer Mag. 11 July 28: And whaddaya know?