This idiom or expression like phrase means:

Good bye.
Take care of yourself.

This idiom or expression like phrase means:

Good bye.
See you later.

This idiom or expression like phrase means:

Good bye.
See you later.

This idiom or expression can best be explained through examples:

Accepting the loss of something.

When someone says “If you left your wallet at the casino you can kiss it good-bye,” they mean something like “Forget about it because you won’t see it anymore. It’s gone!”

Another example: Not doing something anymore.

When someone says “I’ve kissed college good-bye,” they mean something like “I won’t be going to college anymore.”