Hello,
I cannot recall that term, but my mind is getting old. I am only going to guess here, that the reference is to a type of woolen fabric that contains (what we call today) recycled woolen fibers. These fibers could be what was called mungo, which were the cast offs or droppings from the original spinning of the wool. Or the fibers could be type of what was called "shoddy" which was a variety of wool fabrics of various grades all of which contained fibers culled from a previously made garments. It would seem they might use this type of wool fabric for prisoners suits, so that is my guess at this point.
However, this is just a guess. I am sure someone else may come along with more help.
Great question, by the way!