Antonyms for "say one's piece" include concealing, hiding, suppressing, and withholding. Rather than openly expressing oneself, these antonyms suggest keeping thoughts, opinions, or emotions hidden or restrained. In some situations, concealing one's thoughts or feelings may be necessary for diplomacy or tact. However, in other instances, holding back one's ideas or emotions may lead to feelings of frustration or regret. Thus, it is essential to strike a balance between speaking one's truth and being considerate of others. Ultimately, the choice to say one's piece or to conceal one's thoughts is a personal decision that depends on the situation and the desired outcome.