"Having solid evidence," "possessing reliable information," and "holding trustworthy testimony" are all synonyms for the phrase "haddest good authority." They convey the message of being fully informed and confident in the validity of one's sources. It suggests that the information or knowledge held is beyond doubt and carries great weight in any given situation. Having good authority can provide a sense of certainty and security in decision-making, as one is confident in the accuracy of their sources. It shows that one has done their due diligence in obtaining accurate information, making it a valuable tool for anyone seeking to make informed choices.