The phrase "opened one's mouth" can have both positive and negative connotations, depending on the context in which it's used. Some antonyms for "opened one's mouth" could include "kept silent," "remained quiet," "held one's tongue," or "stayed mum." These phrases might suggest that someone chose not to speak up or share their thoughts or feelings about a particular situation, either out of fear, respect, or a desire to avoid conflict. On the other hand, antonyms for "opened one's mouth" could also include "shouted," "yelled," or "screamed," which might suggest a more forceful or aggressive response. Ultimately, the antonym for "opened one's mouth" will depend on the specific situation and the tone that the speaker wishes to convey.