Dioneo's role as the narrator of the "Tenth Day: Tenth Tale.", he tell this particular tale to the other members of the brigata and it reveal about his character of recognizing the gender hierarchy in society and focuses most of his life’s work exploiting the tensions that result.
When comparing the man's legs to rubber in context, it sounds like he is unable to walk. Describing his legs as rubber would indicate they bent and bowed under his weight on them. He's probably either injured or lame.
