What is a kind of highly sized rag paper or board, sometimes containing a small admixture of wood pulp; -- so called because used originally, as now, in presses for pressing and finishing knit underwear called?

pressboard — A kind of highly sized rag paper or board, sometimes containing a small admixture of wood pulp; -- so called because used originally, as now, in presses for pressing and finishing knit underwear. (Webster's 1913.)