Bigmouthed means someone who is excessively talkative and opinionated. Antonyms for this word would include terms that describe someone who is reserved, quiet or reticent. Such antonyms could include words like shy, introverted, timid, subdued, diffident or bashful. Other antonyms for bigmouthed could include adjectives like unassuming, discreet or modest, which describe people who don't draw attention to themselves or seek the limelight. Alternatively, words like considerate, respectful or tactful could also work as antonyms for bigmouthed because they suggest people who are mindful of the impact their words and actions have on others. Overall, the antonyms for bigmouthed are terms that describe people who are the opposite of loud, boastful, or overbearing.