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Is it by any chance the case that, historically, the い-adjective ending 〜かった
is a contraction originating from 〜くあった, where あった is the past inflection of
ある?

To me, it sure _sounds_ plausible, and would be quite elegant given how 〜く and
〜ある seem to stick together in the negative forms of い-adjectives. Is this
where 〜かった originates, or is this merely a coincidence?

**例えば**

  * 高い (?) → 高い
  * 高かった (?) → 高く **あった**
  * 高くない (?) → 高く **ない**
  * 高くなかった (?) → 高く **な** く **あった**
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2019-11-10T19:31:43.430
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question
etymology conjugations i-adjectives
Origin of 〜かった?
> Is it by any chance the case that, historically, the い-adjective ending 〜かった
> is a contraction originating from 〜くあった, where あった is the past inflection of
> ある?

That's exactly what you're seeing.

For ～い adjectives, there were three base conjugation forms:

  * ～し -- 終止形【しゅうしけい】: terminal / conclusive, for ending a sentence of clause.
  * ～き -- 連体形【れんたいけい】: attributive, for modifying a noun or other 体言【たいげん】 (uninflecting word).  
⇒ The ～し and ～き forms are fused in the modern language to just ～い.

  * ～く -- 連用形【れんようけい】: adverbial, for modifying a verb or other 用言【ようげん】 (inflecting word).

Classical grammars often also include:

  * ～けれ -- 已然形【いぜんけい】: often treated as the [realis](https://en.wikipedia.org/wiki/Realis_mood) for things that are definite, or are becoming so, or can become so; superseded in functional terms by hypothetical or conditional usage, resulting in the modern term 仮定形【かていけい】 or "hypothetical / suppositional / subjunctive form".

That said, some authors postulate that the ～けれ ending is also a fusion of ～く +
something else, often suggested as あれ. Personally, I don't see how //ku// +
//are// could possibly become //kere// just on phonetic grounds, so I wonder
if it must have been something else.

You can see a bit more about this in the conjugation table at
[https://ja.wikipedia.org/wiki/形容詞#ク活用](https://ja.wikipedia.org/wiki/%E5%BD%A2%E5%AE%B9%E8%A9%9E#%E3%82%AF%E6%B4%BB%E7%94%A8).
The so-called _-kari_ conjugation pattern clearly shows the ～く + ある fusion.
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2019-11-10T20:07:42.690
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answer
That is indeed correct. The -く form is the 連用形 (infinitive) of the adjectives,
and from the earliest times it had the possibility to attach the verb -ar as
an auxiliary to ease the further conjugation in the verbal categories (such as
negation, mood, aspect). But, with the great reshuffle of the verbal paradigm
during Late Middle Japanese (Ashikaga shogunate), most of these forms went out
of usage. Nevertheless, the past form, newly formed during that period,
employed this intervening verb (Frellesvig, Bjarke. A history of the Japanese
language. Cambridge University Press, 2010. - p. 340 seems to claim that the
appearance of a systematic past for adjectives is already an Edo development),
originally employed without contraction, as -ku atta. There was the alternate
in -ku gozatta at the time. As already mentioned, the contraction happened
later.
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answer