What is the real meaning behind 胸三寸?


「胸三寸に納めるつもりだ」

From what I've read, 「胸三寸」 literally translates to "heart, mind, and feelings"
and 「納める」roughly translates to "to pay" or "to put away".

This line is from 美味しんぼ




[「寸{すん}」](https://ja.wikipedia.org/wiki/%E5%AF%B8)as standardized in the Meiji
era is about 3 cm in the International System of Units (SI.)

「胸三寸{むねさんずん}」means three 寸 (9 cm) into the chest, which is where the heart was
thought to be. 「胸三寸に納める」literally means "to put away deep in the chest". This
phrase according to
[デジタル大辞泉](https://dictionary.goo.ne.jp/word/%E8%83%B8%E4%B8%89%E5%AF%B8%E3%81%AB%E7%B4%8D%E3%82%81%E3%82%8B/)
means:

> 心の中にしまい込んで、顔にも言葉にも出さないでいる。胸三寸に畳む。「何もかも―・めておく」

Namely it means "to bury (feelings, thoughts) (in your heart)", "not to show
it on the outside".

