What’s a “side wife”??
Well, in the really, really good ole days (and thats pre-Alte Heim), she used to be called a concubine (aka a woman who (among others) has the potential to become one’s wife, or who is fulfilling the role of one’s wife in addition to one’s actual wife). Some might argue that R’ Gershon spoiled it for ehrliche yidden in later generations given that most modern frum women would have their beschert sleepiing out back with the dog if they even raised the subject.