What is the nuance of 解{げ}す？

Question

Today in my JLPT text book, I came across the word 解{げ}す.

I know that the kanji 解 means "unravel", and it can be read 解{わか}る, and with that reading it's synonymous with 分{わ}かる, "to understand".

When I looked up 解{げ}す in the dictionary, it also seems to just mean "to understand" when read this way.

Which made me wonder - why use this reading instead of the usual 解{わか}る reading? Does it carry any extra or different nuance?

Is this reading as rare as I think it is, or is it just me that hasn't come across it very often?

Answer 1

Moved to answer from comments.

I mainly encounter this in the potential-negative as 解{げ}せない. As opposed to 解{わか}らない ("don't understand"), 解{げ}せない feels more like the speaker has tried and failed to unravel any kind of explanation, as if the situation seems to defy all logic - "can't fathom/comprehend" feels like a close English equivalent.

It seems to come up a lot in the context of failure to understand people's motivations.

研究社 新和英中辞典 @ Weblio:

彼女がなぜあんなことを言ったのか解せないよ.
We can't understand why she said such a thing.

英辞郎 @ Space Alc:

What is beyond our comprehension is that
解せないのは＜that以下＞です。

Answer 2

The only common usages I can think of are 解{げ}せない and 解熱{げねつ}.