The author puns on the Latin masculine ending -us by associating it with the Old French us "use, custom, habit";
Car cis us est contre nature, mais l'altres seroit par nature. "For this custom [or: this masculine form] is against nature, but the other would be according to nature." Karen Pratt, "Humour in the Roman de Silence", in: Keith Busby, Roger Dalrymple (eds.), Comedy in Arthurian Literature, 2002, p. 94.