"heavy material used to steady a ship," 1520s, from Middle English bar "bare" (see bare (adj.); in this case "mere") + last "a load, burden," from Proto-Germanic *hlasta-, from PIE root *klā- "to spread out flat" (see lade). Or borrowed from identical terms in North Sea Germanic and Scandinavian (compare Old Danish barlast, 14c.). "Mere" because not carried for commercial purposes. Dutch balg-last "ballast," literally "belly-load," is a folk-etymology corruption.