Green’s Dictionary of Slang

great! excl.

an excl. of approval, often in ironic use.

[US]W.J. Kountz Billy Baxter’s Letters 41: Wasn’t it immense where the main lady spurned the leering villain’s gold, and exclaimed with flashing eye, ‘Rags are royal raiment, when worn for virtue’s sake.’ Great!
[US]‘O. Henry’ ‘The Fifth Wheel’ in Strictly Business (1915) 69: ‘Great!’ said Thomas earnestly.
[US]H.L. Wilson Ruggles of Red Gap (1917) 352: ‘Great!’ she exclaimed in a hearty American manner.
[US]Van Vechten Nigger Heaven 120: Great! cried Dick. Now you’re talking.
[US]M. Spillane One Lonely Night 8: Yeah, great.
[UK]A. Sayle Train to Hell 84: Oh, great, great.
[UK]Guardian 1 Oct. 21: Great! How’s he doing? Great! Hey, what’re you doing on the doorstep? Come in! Great!