Perhaps this has been proposed before - I can't be sure as I haven't read all posts from the beginning.
The origin of the Oxford Bags dates back to 1924. Gentlemen's attire was at its most fashionable by the wearing of knickers or knickerbockers as some would say.
These were baggy pantaloon type trousers which tapered in and finished at the knee. Long socks and shoes worn below that.
The "plus" designation refered to the length (in inches) of fabric extended below the knee - Plus Fours, Plus Eights et cetera.
In 1924 the wearing of such items by Oxford students was banned by the University.
The Bags were able to be worn over the top of the knickers to allow fashion standards to be "maintained" if necessarily concealed.