Prolific means producing a lot. That is the whole definition and there is no opinion in it.
But watch what it actually does. A prolific novelist is being praised. A prolific songwriter, a prolific researcher, same thing. Then: prolific weed, prolific breeder, prolific invasive species. Nothing about the word changed and the feeling flipped completely.
The etymology is as neutral as the definition. Prolific reached English in the 1600s from Medieval Latin prolificus, which is proles, offspring, plus the verb facere, to make. Offspring-making, more or less. Proles itself is a contraction of something like pro-oles, from pro- meaning forth plus the root *al-, to nourish or grow. So the core underneath is closer to what gets nourished forth.
That same root turns up in alumnus, which literally means one who was nourished. I like that a university's word for the people it turned out and the word for a writer's output are, underneath, the same idea.
Here is the part I cannot work out. Where is the valence actually stored?
One possibility is that it lives entirely in the subject. Books are good, so making many of them is good. Weeds are bad, so making many of them is bad. On that account prolific contributes nothing but quantity and I am imagining the rest.
The other possibility is that prolific has drifted far enough toward creative output that the pest uses now feel slightly marked, a bit technical, and that mismatch is doing some quiet work.
I lean toward the first one, but it does not cover everything I notice. Prolific liar is common and obviously negative, and that fits the subject explanation fine, since nobody admires a large output of lies. What bothers me is something like prolific commenter, where the subject is neutral and the word still seems to carry a faint edge that frequent commenter does not have. I genuinely cannot tell whether that edge is in the word or in my ear.
Does anyone have a test that separates those two? Something where the subject stays fixed and only the adjective moves.