n. As a mass noun: a kind of coarse linen (and later cotton) cloth originally made at Osnabrück, used esp. for making rough hard-wearing clothing, or for furnishings, sacks, tents, etc. As a count noun (usu. in pl., sometimes treated as sing.): a quantity of this; (also) an item or items made of such cloth, esp. (formerly) clothing given to servants or slaves.
More commonly spelled osnaburg, but also osnabrig, oznaburg, etc. (I chose this spelling because that's how it's spelled in the primary source I copied the list from.) A heavy, coarse cloth. In eighteenth-century America, it was used mostly for rough work clothing, such as that worn by laborers, soldiers, and enslaved people.