かたくな etymology and relation to 固い

Question

The words 頑な{かたくな} (na-adjective, "stubborn") and 固い{かたい} (i-adjective, "hard" or "stubborn") sound similar and have a similar meaning. I was thinking if these words are related to each other through some conjugation that has lost its meaning over time.

I think 「固く{かたく}な」 would be ungrammatical, but it still reads 「かたくな」. I don't recall seeing other na-adjectives that consist of i-adjective continuative form + な (and another な when modifying the following noun). Maybe it is just a coincidence that these words resemble each other.

The only similar "word class" I can think of are words like 「小さい」 →「小さな」and 「大きい」→「大きな」 but they consist of i-adjective stem + な. They are also rentaishi and not na-adjectives.

Are these words related to each other? If not, what is 「かたくな」 based on?

Answer 1

As already pointed out in a comment, the last な of かたくな is not conjugation but a part of the word. You have to put another な to make it attributive.

かたくなな態度

We write out the last syllable as okurigana solely for word identification purpose without grammatical rationale. It's the same way we do for some otherwise single-kanji words like 概【おおむ】ね (< 大【おお】 + 旨【むね】 "(in) general idea") or 自【みずか】ら (< 身【み】 + つ + から "out of self").

かたくな is etymologically from two words: かた and くな. There is a controversy on whether the first portion is from 固 "hard; tough" or 片 "deviant". I personally feel that the latter is more likely, because earliest usages of the word show the sense "having wrong belief", while the "stubborn" sense isn't attested until the 15th century (according to 日本国語大辞典). It however seems that the apparent resemblance to 固い has pulled the meaning all the way to where it belongs today.

The second part くな means (or meant) "twisted, crooked, wicked", which is related to くねる (as in 曲がりくねる) today. 続日本紀 reports that Empress Kōken (孝謙天皇) forced to rename a rebel leader to くなたぶれ "crooked mad (man)" in 757, so we can see the word was in active use at that time.