*úcostima or *úcósima for "undisputable" comes to mind (cp. √KOSO in QL, KOT > KOTH in Etym.), though that's not quite the same (and isn't an adverb).

Yes, I have the feeling Tolkien would come up with some nice idiom for this...

Yeah - and it might be something more along the lines of aqua's derivation instead of a mix of affixes?

As a response to a question, perhaps a simple É náto! 'Yes, indeed!' would do.

Speaking of naturally, nature itself is a question. Perhaps *onnasta 'all things begotten' could be a possibility.

The problem with the word nature is its many diverse meanings. Eldarissa has nostale for 'nature' = 'innate or essential qualities': nostalen mâra 'good by nature' (PE 11:10). Perhaps we could use nostalénen for 'naturally' in the sense 'as can be expected from the nature of something or someone'.

+Björn Fromén
the more I think of your proposition the more I like it. It fits the natus - natura - born - 'everything that's born' - 'something that is born with us' - innate (in-born) trait metaphor nicely.
Sure, the meaning of 'nature' in English (and other languages) got sloppy over time, and nowadays it mostly means 'the world that has not been transformed by humans', including inanimate things.

Yes, as nosta- already meant 'give birth to' in Eldarissa (QL), Tolkien most probably intended nostale to parallel the Latin natura from nasci, natus.

Ideas for S cognate? To use noss or onna?