Positive economics, that is, value-free theory, is contrasted with normative economics which is value-laden.
What is the etymology of positive economics?
Positive doesn't refer to the common sense of optimistic, but to Positivism, a philosophical framework that advocates using empirical data to understand a concept. Positivism subsequently comes from the verb posit, which is to put forth an idea as a matter of fact for the purposes of an argument.
So, it's called positive economics because its based on empirical data as opposed to arguments about morality, and thus is, as you said, value-free.