"Knowing inside out" is a common phrase used to describe someone's deep knowledge of a particular topic. However, there are several synonyms that can be used to convey the same meaning. One possible substitute is "mastering," which implies a thorough understanding and expertise in a subject. "Expertise" can also be used to denote a high level of knowledge and skill. Another option is "intimately familiar," which suggests a deep understanding of the subject. "Well-versed" and "versed" can also convey a strong level of familiarity and experience. Ultimately, the choice of synonym will depend on the specific context and desired tone of the expression.