There are many synonyms for the phrase "hast good authority," which means that one has reliable information or a high level of credibility. Some alternatives include "can vouch for," "has trustworthy evidence," "is a reliable source," "is an expert," or "has a credible reputation." Other phrases that convey a similar meaning include "has a solid track record," "is highly respected," "has proven credentials," and "has earned quite a reputation." When you want to convey that someone is trustworthy or reliable, these synonyms for "hast good authority" can come in handy in order to add variety to your writing and make your message more impactful.