A name coined by Granny Weatherwax, in deep and intense disapproval of cross-dressing, to wit, trousers worn by a woman. As the leggings in question were an uncharacteristically practical move by Magrat Garlick to get around the mobility problems and inadvertent exposure caused by wearing full skirts on a broomstick, Granny coined the word "magrats" to describe them. However, Nanny Ogg took Magrat's side, pointing out that they exposed nothing of all that a decent woman could expose in public, as the trousers got in the way of seeing anything of her legs.
The Roundworld reference is to the common leggings with a double woman's name: Jeans.
Or, perhaps, Bloomers.