Antonyms for the phrase "speaking one's piece" include "being silent," "holding back," "withholding one's thoughts," and "keeping quiet." When a person chooses not to speak their mind or share their opinions, they may feel a sense of restraint or fear of judgment. In contrast, speaking one's piece involves expressing oneself confidently and openly, regardless of the potential response from others. However, it is important to note that there is a time and place for expressing oneself, and doing so tactfully and respectfully can prevent misunderstandings and maintain positive relationships.