noun
concealing one's thoughts; reserve; holding back
Almost always used in negative constructions like 腹蔵なく (without reserve, frankly) or 腹蔵のない (open, unreserved). The word itself is somewhat formal and not used in casual speech.
See also: 腹蔵なく
腹蔵なく意見を述べてください。
Please give your opinion without holding back.
彼は腹蔵のない人柄で、誰からも好かれる。
He has an open personality and is liked by everyone.
率直 means 'frank' or 'straightforward' and is a common adjective. 腹蔵なく is a more formal, set phrase meaning 'without reserve'.
遠慮 means 'restraint' or 'hesitation' out of politeness. 腹蔵なく implies speaking without hiding anything, while 遠慮なく means 'without reserve' in the sense of not holding back out of politeness.
Compound of 腹 (belly, mind) and 蔵 (store, hide). The idea of storing thoughts in one's belly is a traditional metaphor for concealing one's true feelings. The exact historical derivation is uncertain.