expression, noun
what's in one's heart; one's emotional state; what one feels
Refers to one's inner thoughts, feelings, or true emotions, often contrasted with outward appearance.
彼は胸の内をなかなか明かさない。
He rarely reveals what's in his heart.
笑顔の裏に、複雑な胸の内を感じた。
Behind her smile, I sensed complex feelings.
本音 is one's true feelings or real intention, often used in contrast to 建前 (public stance). 胸の内 is a more literary, emotional expression for what one carries in one's heart.
心情 is a more formal, abstract term for one's emotional state or sentiment. 胸の内 is more concrete and personal, literally 'inside one's chest'.
Compound of 胸 (chest, heart) + の (possessive particle) + 内 (inside). Literally 'the inside of one's chest', used figuratively for one's inner thoughts and feelings.