mother-in-law room
n.— «The formal living room has fallen off the map in favor of the Great Room. In some newer homes, (mine included) there is a “living room”—although in my house it is referred to as the “Mother-In-Law Room”—as well as a Great Room.» —“What’s IN and what’s OUT for 2008?” by Sharon McKenna-Nunes Certified Staging Professionals Feb. 18, 2008. (source: Double-Tongued Dictionary)