Generally, used to describe several different things as follows:

1. A person's own excellent physical/mental state.

2. The outstanding physical appearance of another person.

3. A state of being extremely thin

(Since frogs do not have hair, something that is "finer than frog hair" means something that is as thin/fine/excellent as possible).

Got the covid-19 vaccine

When you feel so fine you just can't stand it. Frog hair doesn't exist, so, it must be pretty fine!