Not a silly question at all, especially with the double meaning of the word "muslin."

I have made some practice garments out of muslin, but I have also used various other fabrics that suited my needs. For example, for a knit top, I made a muslin out of some weird polyester knit that I would not want to wear. It really helped! I have also gone the lining-as-muslin route, with a dress I made last year, and with my wedding dress (well, I did do two versions of the bodice out of muslin, but I suppose my lining was my 3rd muslin).
As an example, for the jacket I'm making I'm going to do all the basic pieces that make up the jacket, but not the lining or the bias strips. I think I'm going to use some cotton/poly broadcloth I have laying around - it doesn't have as much body as the corduroy, but I think it will be good enough since the corduroy is lightweight.