By liquid metathesis from Proto-Slavic*moldъ, from Proto-Balto-Slavic*maldas, from Proto-Indo-European*(h₂)moldos(“soft, weak”). Cognates include Old Prussian maldai(“boys”), Latin mollis(“soft, weak”), Ancient Greek βλαδύς(bladús, “weak”), μέλδω(méldō) (transitive)/μέλδομαι(méldomai) (intransitive) and ἀμαλδύνω(amaldúnō, “to weaken, destroy”).
Other Germanic Indo-European cognates evolved from the meaning of soften: English smelt (transitive)/melt (intransitive), German schmelzen, probably Swedish smultron[1], Frankish *smalt (whence French émail).