kanji - 決着 (keri or kecchaku)

This word 「決着」 (settlement/conclusion) is read as けっちゃく. Though lately, I've been digging into some more comics and noticed that there's another reading being used: ケリ.

For example:

• これが最後【さいご】だ。決着【ケリ】を付けよう【つけよう】じゃねぇか。


• いくぞ。ここで決着【ケリ】を付けて【つけて】やる。

Is this a new reading because I don't see it in any JAP to ENG dictionary I reference

Answer

It's 当て字. 決着{けっちゃく} does not have that reading and you won't find it in dictionaries. However, けりをつける is a saying in Japanese which does have an almost identical meaning.

Why do they use [当]{あ}て[字]{じ}?

This study divides the reason into 7 forms:

①
口語の読みを示す
②
外来語の読みを示す
③
英語の略表記の読みを示す

④
スポーツ用語
⑤
代名詞
⑥
言い換え表現
⑦
作品固有の表現

I would say けりをつける is a 口語 form※, i.e. you hear is used when speaking but not in writing. However, manga often depict colloquial conversations, which is why you see it's use here.

※You could also consider けりをつける to be 言い換え表現.

What is happening here, is by blending the spoken and written forms, you get full understanding, but also the reading portrays a real spoken conversation, so the manga doesn't feel very "stiff" like reading a long non-fiction novel.

Here is another study for people who want to read further about Ateji.