Green’s Dictionary of Slang

that’s good! that’s lucky!

1854		Democrat & Sentinel (Ebensburg, PA) 25 Oct. 1/4: ‘That’s it, good, good egg!’ said the humourous man’.
1904		Kipling ‘Their Lawful Occasions Pt II’ in Traffics and Discoveries 138: ‘Good egg!’ quoth Moorshed, and brought his hand down on the wide shoulders.
1914		‘Ian Hay’ Lighter Side of School Life 71: ‘You are going to the San.’ ‘Good egg [...] I shall get off Dummy’s extra after tea!’.
1923		Wodehouse Inimitable Jeeves 18: Good egg!
1930		D. Mackail Young Livingstones 264: White tie and so forth. Good egg!
1960		H.E. Bates When the Green Woods Laugh (1985) 313: ‘Good egg!’ Pop said.
1970		H.E. Bates A Little of What You Fancy (1985) 477: Yes, good egg. I’ll try the champers.