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?

  • 4
    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
    Commented Apr 20, 2013 at 1:55
  • 2
    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
    Commented Oct 25, 2013 at 15:59
  • 母音調和
    – Yang Muye
    Commented Mar 27, 2014 at 16:41

1 Answer 1


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

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]伊豫国与利白祥鹿献奉札方有札志与呂許保志止奈毛 (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.

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

  • 1) Do verbs even have 被服形? Does the reference you mention state this, and does it explain at all how this functions in verbs as compared to nouns? 2) Japanese in the 上代 stage has an indeterminate number of vowel sounds. Some researchers suggest as many as 8, others as few as 3. See ja.wikipedia.org/wiki/… for some discussion of this. 3) With #2 in mind, do you have any information on the 甲・乙 spelling patterns for these purported 被服形 for verbs? Commented Jul 5, 2014 at 8:03
  • 4) All 被服 > 露出 shifts in nouns that I'm aware of involve vowel fronting, such as /ama/ > /ame/, /kutu/ > /kuti/, /ko/ > /ki/. I've never heard the /o/ <> /a/ shift seen in /sira/ > /siro/ and /kura/ > /kuro/ described as 被服 > 露出. In fact, both /o/ and /a/ forms can produce both bound and unbound terms, such as /kura/ (unbound, i.e. 露出) > /kurai/, /kurasu/, /kuranushi/ (bound, i.e. 被服), and /kuro/ (unbound, i.e. 露出) > /kuroi/, /kuromeru/, /kurogane/ (bound, i.e. 被服). These /o/ <> /a/ shifts appear to be a different phenomenon. Commented Jul 5, 2014 at 8:13
  • 1
    @EiríkrÚtlendi 1) Yes, verbs have 被覆形. The reference does state this, but does not explain its function. 2) When I wrote "since 上代", I did so meaning "after 上代". I will update the answer accordingly. I am aware of 上代特殊仮名遣 and the resulting hypotheses, but thank you for the link. 3) All adjectives listed which have examples in 上代 sources are marked with lines indicating the 甲・乙 spelling. I am willing to add these, but I am uncertain as to how best represent them with the formatting available to me here. I am unaware of there is any pattern; one is not mentioned in any source that I have read.
    – rintaun
    Commented Jul 5, 2014 at 8:26
  • 1
    @EiríkrÚtlendi 4) The 白橋【しらはし】 example was my own (there is no discussion of noun 被覆形 and 露出形 in the cited reference) and I could very well be wrong. Thank you for pointing it out. Regarding the /o/ <> /a/ correspondence, I have not come across any literature discussing that, but this all is also pretty far flung from my area of research. I will ask my professor when I get a chance. Thank you for reposting your comment from the old answer.
    – rintaun
    Commented Jul 5, 2014 at 8:31
  • 1
    @EiríkrÚtlendi I recently wrote a paper on this topic for a class, and my professor (the 蜂矢先生 who wrote the paper I references in this answer) wrote in the feedback that 被覆形=末尾がア列・ウ列・オ列甲類・同乙類 and 露出形=末尾がイ列甲類・同乙類・エ列乙類. Also, apparently different vowel changes occur when they are between a 露出形 and 被覆形(山口佳紀氏の言う「異価的交替」) and when they are between 被覆形 and 被覆形(同じく「等価的交替」). I was unaware of any such phenomenon, so that's certainly interesting.
    – rintaun
    Commented Aug 14, 2014 at 7:20

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