The phrase "piece one's mind" essentially means to speak or express one's thoughts or feelings honestly and openly. Antonyms for this phrase might include words like withhold, suppress, conceal, or bottle up. These words imply that someone is intentionally keeping their thoughts or emotions hidden, either for fear of judgment or because they simply do not wish to share. Other antonyms could include words like deceive, dissemble, mislead, or deceive, which would suggest that someone is actively trying to hide or distort their true thoughts or feelings. Overall, the antonyms for "piece one's mind" invoke the idea of secrecy, dishonesty, and a lack of transparency.