expression, noun
close friend; trusted friend; confidant
A friend one can trust deeply, often used in literary or formal contexts. The phrase 腹心 (ふくしん) literally means 'belly and heart', implying someone who knows one's innermost thoughts.
彼は私の腹心の友であり、どんな秘密でも打ち明けられる。
He is my trusted friend, and I can confide any secret in him.
腹心の友と呼べる人はそう多くない。
There aren't many people you can call a close friend.
心友 is a less common synonym for a close friend, similar in warmth to 親友, but 腹心の友 carries a stronger nuance of being a confidant.
Compound of 腹心 (ふくしん, 'one's innermost thoughts; trusted aide') + の (possessive particle) + 友 (とも, 'friend'). The term 腹心 originally referred to the belly and heart as the seat of one's true feelings, later extended to mean a person one trusts completely.