You UL fans thought you were going to get something real juicy, didn’t you? Well, actually, all I want to know is how this expression came about.
IIRC, Jan Harold Brunvand coined it. He’s a folklorist. He’s written several books, collections of urban legends. The Vanishing Hitchhiker was the first, and goes into a lot of detail about how the term “urban legend” came into being, and what his definition of UL is.
Unless, of course, you were looking for an explanation of the term “the mother.” can’t help ya there.