Green’s Dictionary of Slang

all right! everything is in order!

1934		J. O’Hara Appointment in Samarra 60: ‘[S]tay there till they close up and keep an eye on things, see what I mean? [...]’ ‘Kay,’ said Al. ‘Only too glad, Ed’ .
1950		C. Harris Three-Ha’Pence to the Angel 154: ‘Reply?’ he said [...] ‘Eh? No. No. ’Ere, wait a minute ... No, no, thass all right.’ ‘Kayee,’ said the boy.
1978		S. Longstreet Straw Boss (1979) 251: Moz is kay-okay. He’s a real legal whiz.
1985		J. Wambaugh Secrets of Harry Bright (1986) 144: ‘Kay by me,’ Sidney Blackpool said.
1997		C. Hiaasen Lucky You 89: ‘We ain’t never met, you and me, far as the lottery bureau is concerned.’ ‘’K.’.