I remember being told this version years ago in a french class so forgive me if my spelling is wrong... The word 'trouser' was derived from the original practice of clothing the legs in pieces of fabric bound on with string or rope, literally 'trussed' or (as I said, forgive me if the spelling is wrong) 'trousse' from the french 'to bind'. If the legs were bound individually, there would have originally been 2 separate garments. This explanation is the most feasible I've heard so far...