One's heart or inner feelings; often used in a somewhat literary sense to refer to opening one's mind or feelings.
彼女は胸襟を開いて本音を話した。
She opened her heart and told the truth.
友人に胸襟を打ち明けることができて、気持ちが楽になった。
Being able to confide in a friend relieved me.
首脳は会談で胸襟を示した発言をした。
At the talks the leader made remarks that showed openness of mind.