The term "be unfamiliar with" can be replaced with a variety of synonyms such as "not know," "be ignorant of," "be uninformed about," "be unaware of," "be unacquainted with," or "be new to." Each of these synonyms carries a slightly different connotation, but they all refer to a lack of familiarity or knowledge with a particular subject or topic. These synonyms can be useful when trying to communicate a gap in understanding or expertise, and can help to clarify the level of knowledge or experience a person has with a given subject. Ultimately, the choice of synonym will depend on the context and intended meaning of the statement.