Possibly a substratum word, akin to Albanian bulëz. Numerous other theories exist, none of which are satisfactory: a link to Russian бу́лка (búlka), Polish bułka; or from a Vulgar Latin *bulgius, from Latin buga (cf. French bouge, Italian bolgia); or a Vulgar Latin *balteum; or *bullidus, from bulla (as with solz from solidus); or Albanian *buljëzë; or a Gepid *bultia. It may be derived from a German word (cf. Spanish bulto) through a Slavic intermediary, cf. Russian булыжник (bulyžnik). It is more likely an internal derivation from bolf, itself of Slavic origin or an expressive creation, through a form *bulh, bulg with plural bulgi that was remade into a singular.