Green’s Dictionary of Slang

as affirmative, that’s absolutely right; I don’t disagree at all, I know only too well; occas, as interrog.

1931		W.R. Burnett Silver Eagle 146: Harworth said: ‘She’s a wonderful girl.’ Larsen looked at him. ‘Are you telling me?’.
1935		Operator 1384 Devil’s Diplomats 27: [as interrog.] ‘I warn you, McCann, that Sidi is a dangerous man with a knife,’ smiled Levine. ‘You’re telling me?’.
1942		Whizzbang Comics 14: ‘You’re telling me,’ laughs Red.
1952		A.W. Upfield New Shoe 139: ‘You would go crook if I ordered you to shift all those logs right outside this pub.’ ‘You’re telling me,’ snorted Moss.
1965		(con. 1930s) R. Wright Lawd Today 134: ‘That nigger’s sick.’ ‘You telling me?’.
1983		S. King Christine 450: ‘But they don’t look anything alike.’ ‘You’re telling me,’ I said.
1999		Indep. Rev. 25 Aug. 12: ‘You’re telling me,’ I thought, but said nothing.