Green’s Dictionary of Slang

1. (US black) an exclamation of amazement, usu. ironic.

1965

H. Rhodes Chosen Few (1966) 179: ‘Ain’t that late!’ said Tan.

2. (US teen) see you later!

1997–2000		Da Bomb 🌐 17: Late: Short for ‘See you later.’.
2002		Teen Lingo: The Source for Youth Ministry 🌐 late (derived from ‘later’) Something to say when leaving, as if to say ‘I’m out of here.’.