Stuff is the old English word for a thick, coarsely woven worsted wool and then possibly for linsey – woolsey ( or Woolsey-linsey) a wool/linen mix.
At court, lawyers’ gowns were called stuff in contrast with that worn by the Queen’s Counsel, which are made of silk.
Stuff subsequently became the general term for all woven fabrics, and now it’s a bit like ‘frock’–a word still just about in use but only by the outgoing generations. For the Jewish community linen/ wool mix is called Shatnez.