one's innermost thoughts; one's true feelings
Literary and somewhat formal; used to express one's humble, sincere inner feelings, often in written or formal contexts.
I am grateful for the opportunity to express my innermost thoughts.
His letter revealed his true feelings.
Compound of 微 (び, 'humble, slight') and 衷 (ちゅう, 'innermost, heart'). The exact historical derivation is uncertain, but the spelling conventionally conveys modest inner feelings.