The phrase "speak one's piece" means to express one's opinion or point of view in a clear and articulate manner. The antonyms for this phrase would be words or phrases that indicate a lack of expression or communication. For example, phrases like "hold one's tongue," "remain silent," or "keep mum" would all be antonyms for "speak one's piece." Other antonyms could include phrases that suggest hesitance, uncertainty, or a lack of conviction, such as "beat around the bush," "equivocate," or "dance around the issue." In contrast, synonyms for "speak one's piece" might include "voice one's thoughts," "articulate clearly," or "make one's point known.