i-adjectives that end in a 〜ない which doesn't seem to be 〜無{な}い

Question

I noticed there are many 形容詞{けいようし} (i-adjectives) that end in 〜ない, where the な is not part of the kanji, and doesn't seem to have the meaning 無い.

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Examples:

• 危{あぶ}ない means "dangerous", while 危 means "danger"
• 少{すく}ない means "few"/"scarce", while 少 means "few"/"little"
• 切{せつ}ない means "heartrending"/"trying", while 切 means "earnest"/"ardent"

(For contrast, examples where it's clearly 無い: 心ない, 情けない, 力ない.)

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I wonder where exactly this ない is coming from. It is not a classical Japanese ending to my knowledge (like 〜しい).

I speculate that it could be an alternative shortening of the classical Japanese copula 〜なり, but this is odd to me because I suspect the usual shortening of 〜な would be used if this were the case (as we see in 形容動詞{けいようどうし} (na-adjectives)).

Does anyone know the etymology of these words and/or the meaning of this 〜ない?

Answer 1

Thanks to snailplane's and Dono's links, it seems that the answer is fairly established:

大辞泉

形容詞・形容動詞の語幹など性質・状態を表す語に付いて形容詞をつくり、その意味を強調する

大辞林

性質・状態を表す語（形容詞・形容動詞の語幹など）に付いて形容詞をつくり、程度のはなはだしい意を表す

Namely, 〜ない is also a suffix that attaches onto words describing state or quality, turns them into a 形容詞, and emphasizes them.