Why do we say 頼もしい instead of 頼ましい?


In a [previous
question](https://japanese.stackexchange.com/questions/11678/what-is-
the-%E3%82%8F-in-%E5%BF%8C%E3%81%BE%E3%82%8F%E3%81%97%E3%81%84-and-%E5%98%86%E3%81%8B%E3%82%8F%E3%81%97%E3%81%84),
I posted a list of adjectives produced from verbs using the
[`しい`](http://dictionary.goo.ne.jp/leaf/jn2/92673/m0u/%E3%81%97%E3%81%84/)
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`](http://dictionary.goo.ne.jp/leaf/jn2/10982/m0u/%E5%8B%87%E3%81%BE%E3%81%97%E3%81%84/)  
>  悩む → 悩ま ＋ しい [`nayam-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/165172/m0u/%E6%82%A9%E3%81%BE%E3%81%97%E3%81%84/)  
>  喜ぶ → 喜ば ＋ しい [`yorokob-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/228643/m0u/%E5%96%9C%E3%81%B0%E3%81%97%E3%81%84/)  
>  妬む → 妬ま ＋ しい [`netam-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/170180/m0u/%E5%AB%89%E3%81%BE%E3%81%97%E3%81%84/)  
>  呪う → 呪わ ＋ しい [`norow-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/172717/m0u/%E5%91%AA%E3%82%8F%E3%81%97%E3%81%84/)  
>  慕う → 慕わ ＋ しい [`shitaw-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/97752/m0u/%E6%85%95%E3%82%8F%E3%81%97%E3%81%84/)  
>  好む → 好ま ＋ しい [`konom-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/81099/m0u/%E5%A5%BD%E3%81%BE%E3%81%97%E3%81%84/)

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`](http://dictionary.goo.ne.jp/leaf/jn2/81117/m0u/%E5%A5%BD%E3%82%82%E3%81%97%E3%81%84/)  
>  狂う → 狂お ＋ しい [`kuruw-o-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/64329/m0u/%E7%8B%82%E3%81%8A%E3%81%97%E3%81%84/)  
>  頼む → 頼も ＋ しい [`tanom-o-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/138149/m0u/%E9%A0%BC%E3%82%82%E3%81%97%E3%81%84/)

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`](http://dictionary.goo.ne.jp/leaf/jn2/64329/m0u/%E7%8B%82%E3%81%8A%E3%81%97%E3%81%84/)
> [`kuruw-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/64502/m0u/%E7%8B%82%E3%82%8F%E3%81%97%E3%81%84/)  
>  [`konom-o-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/81117/m0u/%E5%A5%BD%E3%82%82%E3%81%97%E3%81%84/)
> [`konom-a-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/81099/m0u/%E5%A5%BD%E3%81%BE%E3%81%97%E3%81%84/)  
>  [`tanom-o-
> sii`](http://dictionary.goo.ne.jp/leaf/jn2/138149/m0u/%E9%A0%BC%E3%82%82%E3%81%97%E3%81%84/)
> `*tanom-a-sii` (I can't find evidence that ×頼ましい exists)

So how can these `-o-` forms be explained?




I answered this question a while back, but unfortunately I only half-
understood it, and there was some wrong information in it. After some time
researching this question, and now an entire semester of a course about
Japanese adjectives, I am ready to answer it again. As such, I have deleted my
previous answer.

So, since しい appears to attach to the 未然形 of a verb, why with some small
subset, does it attach to the -o- form of the 未然形 rather than the -a- form?

That is because **しい does not attach to the 未然形 of a verb**.

Rather, **it attaches to the 被覆形【ひふくけい】 1 of a verb**. Most often, this form
resembles the 未然形 -a-2, but there are a number of other examples。Some examples
which still exist in modern Japanese include: `乏【とぼ】しい` `恐【おそ】ろしい` `恨【うら】めしい`
`宜【よろ】しい`

As far as I can tell (and indeed, as far as I can find in the literature),
there is not a solid rule for determining the 被服形 of a verb. -a- is the most
common, e.g. 喜【よろこ】ぶ→喜【よろこ】ば + しい. However, as seen in the examples above, for
words in which the previous syllable ends in -o (ぼ, そ, ろ, の, etc.), there is a
trend toward the 被覆形 also ending in -o.3

* * *

Notes:  
1 The 被覆形 is essentially a linking form. It is most often seen in reference to
compound nouns, e.g. 雨【あめ】 + 雲【くも】 = 雨雲【あまぐも】 (め→ま) and 白【しろ】 + 橋【はし】 =
白橋【しらはし】 (ろ→ら）. The opposite of 被覆形 (i.e. the form when used as a standalone
word) is 露出形【ろしゅつけい】.

2 This is perhaps easily understood because (at the very least after 上代),
Japanese only has 5 vowels and it will thus almost certainly look like
_something_.

3 In fact, modern 喜ばしい was also originally *喜ぼしい. e.g.

> **ヨロコ** ボシ［悦］伊豫国与利白祥鹿平献奉天在札方有札志与呂許保志止奈毛見流 （四十六詔・続紀神護景雲三［769］年）
>
> `伊豫国【いよのくに】より白【しろき】祥【しるし】鹿【しか】を献奉【たてまつり】て在【あ】ればうれし よろこぼし となも見る`

Note: The example above was originally printed vertically, with the bold
characters having a vertical line to the left.

* * *

References:  
蜂矢真郷(2012)「上代の形容詞」『萬葉』(萬葉学会) 第212号, pp. 1-35

