If you speak or act familiarly, you do it in a warm, informal way — as if you already know the person well. Friends use nicknames, a light touch on the arm, the kind of jokes you would not try on a stranger. That can feel kind, or it can feel too close: a shop assistant who talks familiarly to a customer may sound friendly to one person and rude to another. There is a second, more written use: familiarly known as, when you give the name people actually use — “William, familiarly known as Bill.” That one is a little old-fashioned and sits in biographies more than in speech. The word is about closeness of manner, not about whether two things look the same.