- cassock (n.) mid-16c., from M.Fr. casaque "long coat" (26c.), probably ultimately from Turk. quzzak "nomad, adventurer," (the source of COSSACK (Cf. Cossack)), from their typical riding coat. Or perhaps from Arabic kazagand, from Pers. kazhagand "padded coat," from kazh "raw silk" + agand "stuffed."
Etymology dictionary. 2014.