Muslin is a kind of cotton fabric that's plain and lightweight. Designers and dressmakers often use muslin to make inexpensive sample garments for fitting.
Perfecting designs using muslin is so common that the practice garment itself is often referred to as a muslin: "Here, try on this muslin before we cut the pattern out of the silk." The word muslin comes from the French mousseline, which is rooted in Mosul, the city in Iraq where Western traders first encountered the fabric. This ancient muslin was a luxurious material, while today's is much more ordinary and affordable.