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In a previous question, I posted a list of adjectives produced from verbs using the しい suffix. In each example, it seems that しい attaches directly to the 未然形:

勇む  →  勇ま + しい    isam-a-sii
悩む  →  悩ま + しい    nayam-a-sii
喜ぶ  →  喜ば + しい    yorokob-a-sii
妬む  →  妬ま + しい    netam-a-sii
呪う  →  呪わ + しい    norow-a-sii
慕う  →  慕わ + しい    shitaw-a-sii
好む  →  好ま + しい    konom-a-sii

In all of the above words, it appears that the 未然形 has the -a- surface form.

However, in these three words, it seems that it has the -o- surface form:

好む  →  好も + しい    konom-o-sii
狂う  →   狂お + しい    kuruw-o-sii 
頼む  →  頼も + しい     tanom-o-sii

What I'd read previously is that -a- and -o- are both considered the same underlying form because of the sound change /au/ → /o:/. In other words, the -o- surface form is underlyingly -a-, but changes to -o- as part of /o:/.

However, these three adjectives do not contain the long vowel /o:/, so I don't think that explains why they have an -o-. And if this is the result of a sound change, it doesn't appear to be a regular sound change, because most adjectives have -a-. In fact, both konom-o-sii and kuruw-o-sii have -a- versions:

kuruw-o-sii   kuruw-a-sii   
konom-o-sii   konom-a-sii   
tanom-o-sii   *tanom-a-sii   (I can't find evidence that ×頼ましい exists)

So how can these -o- forms be explained?

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There is difference in meaning between 狂おしい and 狂わしい. There is also difference in meaning between 好ましい and 好もしい. But I didn't know the difference until I read this article. Though I thought about and looked into 頼もしい, I couldn't find the reason. I am looking forward to some answers to be posted from now on. –  kerochan Apr 20 '13 at 1:55
Another to add to the list: 愛【いと】しい. According to Daijisen, it is derived from 厭【いと】う (originally as 愛【いと】おしい). 厭【いと】わしい also exists. –  rintaun Oct 25 '13 at 15:59
母音調和 –  Yang Muye Mar 27 at 16:41
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1 Answer

I had the chance to discuss this briefly with the professor of a Japanese Linguistics course I'm taking at the moment, and it turns out that he is quite knowledgeable on this particular subject. That said, it was a rather short conversation, and any shortcomings in this answer are certainly my own.

According to him, the ~しい adjectives, generally speaking, are in fact formed from the 未然形 of their originating verbs. Additionally, the 未然形【みぜんけい】 of a verb most of the time ends with -a. However, that is apparently not always the case, and there are in fact instances of the 未然形 of a verb ending with each of the five vowels.

Though the -ashii verbs are numerous, there are many other counterexamples than just the three above -- some of which are very difficult to explain, or are not well understood. Based on my own research, うれしい is one such example.

頼【たの】もしい, as it turns out, is quite easily explained. The reason that it ends with -o instead of -a is that it is preceded by の. This can be seen in 好【この】もしい as well, though I do not know why 好【この】ましい exists as well. That also does not explain 狂【くる】おしい.

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My professor expressed interest in continuing the conversation if I desired to, so if you'd like a more detailed answer, leave a comment and I'll try to set something up so I can flesh it out some more. –  rintaun Oct 24 '13 at 6:48
I would greatly appreciate feedback on my answer, which I could potentially use to improve it (as well as my future answers) -- instead of just a downvote. –  rintaun Oct 24 '13 at 7:40
I am not sure why the 未然形 explanation makes sense. Nobody says 好もない. –  user54609 Oct 24 '13 at 18:11
@rintuan But usually the -o- form only occurs as part of the sound change /au/ → /o:/ (as in your example 書かう → 書こう), and that doesn't seem to apply here because there's no /au/. I guess I was wondering if this was a different phenomenon for that reason, or if it could be somehow related to that sound change. –  snailboat Oct 25 '13 at 15:36
@rintaun It's been a month... –  user54609 Jan 1 at 7:01
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