1. something you say to somebody who is bothering you and you don't want to talk to him or her. Comes from "Later with you" or "Later with this jerk." And if the guy is a REAL jerk you tell him, "Later with you....MUCH later." If you say "lates" it's a mild way to say it.
2. something you say when something bad happens
1. Somebody says something stupid, you say, "Lates" means go away.
2. You miss the bus, you say "Ooh, lates!"