Why is the ninth month called September?
c.1600, "delicately woven cotton fabric," from French mousseline (17c.), from Italian mussolina, from Mussolo, Italian name of Mosul, city in northern Mesopotamia (modern Iraq) where muslin was made. Like many fabric names, it has changed meaning over the years, in this case from luxurious to commonplace. In 13c. French, mosulin meant "cloth of silk and gold." The meaning "everyday cotton fabric for shirts, bedding, etc." is first attested 1872 in American English.
plain-woven cotton fabric made in various weights. The better qualities of muslin are fine and smooth in texture and are woven from evenly spun warps and wefts, or fillings. They are given a soft finish, bleached or piece-dyed, and are sometimes patterned in the loom or printed. The coarser varieties are often of irregular yarns and textures, bleached, unbleached, or piece-dyed and are generally finished by the application of sizing. Grades of muslin are known by such names as book, mull, swiss, and sheeting.