etymology - What is the わ in 忌まわしい and 嘆かわしい?

On chat, Chocolate helped me find some examples of adjectives produced from verbs using the しい suffix. In the following examples, it appears to attach directly to the 未然形:

勇む　　→　　勇ま　＋　しい
悩む　　→　　悩ま　＋　しい
喜ぶ　　→　　喜ば　＋　しい
妬む　　→　　妬ま　＋　しい
呪う　　→　　呪わ　＋　しい

慕う　　→　　慕わ　＋　しい
好む　　→　　好ま　＋　しい
好む　　→　　好も　＋　しい
頼む　　→　　頼も　＋　しい

But in these last two examples, there seems to be an extra わ inserted:

嘆く　　→　　嘆か　＋　わ　＋　しい
忌む　　→　　忌ま　＋　わ　＋　しい

I can't seem to find a dictionary entry for わ or わしい. What is this わ?

Answer

The わ in these words is actually the 未然形 of the 継続の助動詞「ふ」, which historically attached to the 未然形 of other verbs. In this case, the combination of 嘆く and ふ formed the verb 嘆かふ, and the combination of 忌む with ふ produced 忌まふ. It is these words that combined with the し suffix:

嘆かふ　　→　　嘆かは　+　し
忌まふ　　→　　忌まは　+　し

In modern Japanese, the は becomes わ, and the classical 終止形 し is replaced with the 連体形 しき, which loses its /k/ and becomes しい:

嘆かは　+　し　　　→　　　嘆かわ　+　しい

忌まは　+　し　　　→　　　忌まわ　+　しい

So as you can see, these words aren't really exceptions. Your list includes 呪う and 呪わしい, and that word can be explained in the same way. And in all of these examples, the suffix しい is attaching directly the 未然形 of a verb.

By the way, there's some interesting discussion in English of the auxiliary ふ in this blog post on tumblr.