You might not have heard the word “familect” before, but chances are you’re already fluent in it. Does your family have an unusual word for the remote control that outsiders wouldn’t recognize? Do you have a nickname that everyone at the office calls you? Does your friendship group have a catchphrase that’s followed you through the years? 

Familect—the idiosyncratic use of language that is particular to a distinct social group, not necessarily a family—is a common linguistic phenomenon. When individuals spend a lot of time together, the language they use changes: Words born from a particular joke or incident slip into common parlance; the rules of grammar and pronunciation are bent and broken; we speak, effectively, in a kind of code. Years ago, an Italian friend and I stay...