Is it ensure, insure, or assure?
"heavy material used to steady a ship," 1520s, from Middle English bar "bare" (see bare; in this case "mere") + last "a load, burden," or borrowed from identical terms in North Sea Germanic and Scandinavian (cf. Old Danish barlast, 14c.). "Mere" because not carried for commercial purposes. Dutch balg-last "ballast," literally "belly-load," is a folk-etymology corruption.