The phrase "know inside out" refers to having complete and thorough knowledge about an area or subject. Some synonyms for this phrase include "know like the back of one's hand" which implies familiarity, "know backward and forward" which implies a deep understanding, and "know inside and out" which is essentially the same as the original phrase. Additionally, "have down pat" or "have mastered" express a level of expertise. Other similar expressions include "be an expert in," "be well-versed in," "be knowledgeable about," and "be acquainted with." No matter which phrase we use, it indicates a high degree of understanding and awareness of a particular subject.