noun
one's heart; one's mind; one's bosom
Literary word for one's inner thoughts, feelings, or heart. Not used in everyday conversation; appears in written language, poetry, or formal contexts.
彼の胸裏に去来する思いは複雑だった。
The thoughts passing through his mind were complicated.
その言葉は深く私の胸裏に刻まれた。
Those words were deeply engraved in my heart.
Compound of 胸 (chest, heart) and 裏 (inside, back), literally 'inside the chest'. The exact historical derivation is uncertain, but the word has been used in literary Japanese to refer to one's inner thoughts and feelings.