What is the precise meaning of "Pretty Good"?

The precise meaning of “Pretty good”? Good, to an imprecise degree.

I wouldn't say it has a flavour of sarcasm, but a flavour of “more or less”.

So when you answer “good” then everything is well and truly definitely good but when you say “pretty good” you're just less than sure about the well and truly definitely part. “Pretty good” is more non-committal.

"Pretty Good", to me, means that things are going basically as expected / hoped, you have nothing major to complain about but nothing major to exult about either. It's a very middle-of-the-road sort of response.

"Good" generally means that you are happy about something, or perhaps are just feeling better than average overall even if you don't know exactly why.

"Very good" and "great" and "awesome" almost always have some specific thing that really went surprisingly well for you to cause such an elevated level of happiness.

It all depends on how it's said. "Pretty good" as a response to a question can mean anything from "really awesome" to "so-so" to "not so hot" or even "could be better" — you just have to have an ear for it.