Is credulus a diminutive?

Question

In the comments to an answer involving the adjective credulus the question has arisen if this word is to be parsed as a diminutive, even if the form of which it would be a diminutive (say, *credus, like fidus) isn't attested. On the other hand, there are adjectives like tremulus and especially sedulus that don't seem to involve a sense of smallness.

So, is the -ulus suffix in credulus a trace of a diminutive or not? Is the answer the same for the other adjectives mentioned?

I have seen that there are other questions on apparent diminutives (angulus, oculus, ulula), but it doesn't seem that the answer to the present question can be extrapolated from those.

Answer 1

According to Gildersleeve and Lodge, §182.1, ‑ulus is a regular former of adjectives from verbs. This particular formation indicates repeated action. They cite querulus and I could add bibulus. ‑bundus is unrepeated action (cunctabundus, nauseabundus). ‑bilis (amabilis, bibibilis, vendibilis) is passive action, as also, I suspect, is ‑ulis (edulis).

The whole section is well worth reading. I used to envy Sanskrit for its systematic approach to verbal adjectives, but Latin, though more chaotic, has nothing to be ashamed of.

And yes, the phonetic coincidence with diminutives can give an "atmosphere" to certain ‑ulus adjectives – a certain air of condescension – but no more than that.

Answer 2

Even though a non-diminuitive version of credulus does not exist, it kind of has this "smallness" connotation. If somebody is credulus, he is childish in some way, so you call at least some part of his brain little developed. This naivity comes with a childish innocence and also dinkiness. Furthermore, this is meant derogatively, so there are some fulfilled aspects of a diminuitive which surely caused this ending. However, gramatically it is still no diminuitive as it does not have an non-diminuitive origin.