etymology - 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?

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

Answer

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• ～けれ -- 已然形【いぜんけい】: often treated as the realis for things that are definite, or are becoming so, or can become so; superseded in functional terms by hypothetical or conditional usage, resulting in the modern term 仮定形【かていけい】 or "hypothetical / suppositional / subjunctive form".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. The so-called -kari conjugation pattern clearly shows the ～く + ある fusion.