Imagine that there is a couple getting ready to go out for the evening. The woman is taking a long time to sort out her clothes, her hair, her make-up etc. The man has been ready for half an hour and is simply waiting for his wife. She finally appears at the bottom of the stairs and says "OK, are you ready?" He replies "Oh, I'm past ready!"
It's a slightly sarcastic reply in this context, but he means he was ready a little while ago and has just been waiting for her.