The term 'advance furtively' can be summarized as the act of sidling, which is a type of movement where one moves sideways or obliquely. It is often used to describe a person who is trying to move around without being noticed. Sidling is a form of stealthy movement that is used to avoid detection or to sneak up on someone. It is also used to describe a person who is trying to get closer to something without being seen.