What is in one's heart; one's inner feelings or thoughts, often those kept private.
彼女の胸の内を知りたくて、何度も話を聞いた。
Wanting to know what was in her heart, I talked to her many times.
胸の内に秘めた思いを手紙に書いた。
I wrote down the feelings I kept in my heart in a letter.
表面は平静でも、胸の内では不安が渦巻いていた。
Even though she appeared calm on the surface, unrest swirled within her heart.
彼の胸の内を打ち明けてもらって、やっと理解できた。
After he finally opened up about his feelings, I was able to understand him.