OMG get over it. They’re called rags. Have been and will forever be called that.There are plenty of songs about utd, not one mentioned the "Rags", not in the last 50 years, even the CDs recorded which were full of abuse for that team. It shows it isn't the common phrase used by City fans.
I could imagine people thinking it is who read fanzines (or did) or go on forums, and then using it themselves. Same with the Swamp, never heard it called that until I read KotK.