intuitively, why do かねる and かねない roughly mean "cannot" and "can"?


I understand the grammatical usage of these words, but I don't understand
_why_ they (roughly) mean "cannot" and "can".

For example, if we write them with kanji, we get 兼ねる and 兼ねない. According to
jisho.org, that kanji has the following meanings: "concurrently, and,
beforehand, in advance". None of these meanings directly corresponds to the
meaning of "cannot" and "can", so why would that kanji be used?

Here's the research so far:

  * There might be a relationship with 悲しい: <http://gogen-allguide.com/ka/kanashii.html>
  * There might be a relationship with 嫌: <https://ja.wiktionary.org/wiki/%E5%85%BC#%E6%B4%BE%E7%94%9F%E5%AD%97>
  * There might be a relationship with 予ぬ: <https://ja.wiktionary.org/wiki/%E3%81%8B%E3%81%AC>

I'm trying to find an intuitive way to understand how this usage came to be,
to better "feel" the intention of the word. Any help appreciated!




First of all, the auxiliary-かねる, or かぬ in archaic Japanese, meaning "cannot",
is almost never written with the kanji 兼 today. I personally have never seen
something like 言い兼ねる, and I have believed the verb 兼ねる and the auxiliary-かねる
are two different words.

Second, the history of auxiliary-かぬ is **very** old, and even
[万葉集](https://en.wikipedia.org/wiki/Man%27y%C5%8Dsh%C5%AB) has examples of it.

> 世間乎 宇之等夜佐之等 於母倍杼母 飛立 **可祢** 都 鳥尓之安良祢婆  
>  (飛立 **可祢** 都 = とびたち **かね** つ = 飛び立てない)

This means it's almost impossible to trace the correct etymology of auxiliary-
かねる. Still, there seems to be a few speculations:

  * Auxiliary-かねる may be etymologically related to 悲し(い) because they share the nuance of "that's beyond my control."
  * Auxiliary-かねる may be etymologically related to [予ぬ【かぬ】](https://ja.wiktionary.org/wiki/%E3%81%8B%E3%81%AC#%E5%8B%95%E8%A9%9E%EF%BC%9A%E4%BA%88) because they share the nuance of "something bad is anticipated."
  * Auxiliary-かねる may be etymologically related to 兼ぬ because they share the meaning of "two situations happening simultaneously (i.e., ideal vs reality)". (This is 時代別国語大辞典's speculation, from [here](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q14189009444))

I have no idea how true these are, but I can at least say these are far from
_intuitive_ :)

Anyway, I think it's best to treat the auxiliary-かねる as something unrelated to
兼ねる meaning "to have more than one role."

Reference:

  * [日本語の質問に答える ～カネル と ～カネナイ](http://blog.livedoor.jp/norayuri/archives/50826667.html)
  * [「～できない」の意の「～かぬ」は何故、動詞ではなく接尾語なんですか？](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q14189009444)

(EDIT: Added one more speculation regarding the etymology of auxiliary-かぬ,
although I personally think it's the least convincing. Most 古語辞典 regard the
two かぬ as two distinct words.)




First of all, the auxiliary-かねる, or かぬ in archaic Japanese, meaning "cannot",
is almost never written with the kanji 兼 today. I personally have never seen
something like 言い兼ねる, and I have believed the verb 兼ねる and the auxiliary-かねる
are two different words.

Second, the history of auxiliary-かぬ is **very** old, and even
[万葉集](https://en.wikipedia.org/wiki/Man%27y%C5%8Dsh%C5%AB) has examples of it.

> 世間乎 宇之等夜佐之等 於母倍杼母 飛立 **可祢** 都 鳥尓之安良祢婆  
>  (飛立 **可祢** 都 = とびたち **かね** つ = 飛び立てない)

This means it's almost impossible to trace the correct etymology of auxiliary-
かねる. Still, there seems to be a few speculations:

  * Auxiliary-かねる may be etymologically related to 悲し(い) because they share the nuance of "that's beyond my control."
  * Auxiliary-かねる may be etymologically related to [予ぬ【かぬ】](https://ja.wiktionary.org/wiki/%E3%81%8B%E3%81%AC#%E5%8B%95%E8%A9%9E%EF%BC%9A%E4%BA%88) because they share the nuance of "something bad is anticipated."
  * Auxiliary-かねる may be etymologically related to 兼ぬ because they share the meaning of "two situations happening simultaneously (i.e., ideal vs reality)". (This is 時代別国語大辞典's speculation, from [here](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q14189009444))

I have no idea how true these are, but I can at least say these are far from
_intuitive_ :)

Anyway, I think it's best to treat the auxiliary-かねる as something unrelated to
兼ねる meaning "to have more than one role."

Reference:

  * [日本語の質問に答える ～カネル と ～カネナイ](http://blog.livedoor.jp/norayuri/archives/50826667.html)
  * [「～できない」の意の「～かぬ」は何故、動詞ではなく接尾語なんですか？](https://detail.chiebukuro.yahoo.co.jp/qa/question_detail/q14189009444)

(EDIT: Added one more speculation regarding the etymology of auxiliary-かぬ,
although I personally think it's the least convincing. Most 古語辞典 regard the
two かぬ as two distinct words.)

