To be a master of something is to have complete knowledge and expertise in a particular field. There are several synonyms for the word "be master", including "be skilled," "be proficient," "be adept," "be expert," and "be knowledgeable." These words all signify a deep understanding and high level of proficiency in a particular area or subject matter. When someone is described as a master, it usually conveys a great deal of respect due to their superior skills and knowledge. So whether you want to describe someone as being proficient in a particular field or an expert who has mastered their craft, these synonyms for "be master" will come in handy.