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.
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.
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.