The phrase "haddest big mouth" is a colloquial expression that refers to someone who talks too much, especially about things they do not know much about or should keep private. Some synonyms for this phrase could include "talks loosely," "gossipy," "blabbermouth," or "chatterbox." Another term to describe someone with a "big mouth" could be "verbose," which means they use too many words to express themselves. Other potential synonyms include "loudmouth," "windbag," or "nosey," depending on how one wants to characterize the person in question. Overall, there are many different ways to describe someone with a "haddest big mouth," so it is helpful to choose the term that best fits the situation.