Mid 19th century. Probably from the unattested Norn reflex of the early Scandinavian word represented by Old Icelandic mǫrueldr, maurueldr (Icelandic maurildi), Faroese mureldur, Norwegian moreld, maureld, Swedish mareld, Danish regional marild, Danish morild. In Icelandic and Norwegian the word has a wider application, ‘phosphorescence seen on the sea, or on rotten wood, raw fish, etc.’, which suggests that the first element of this compound is probably cognate with Norwegian regional mor something that is mouldy or weatherbeaten, maren rotten, decayed (with reference to the phosphorescent light which emanates from decomposed matter); although folk etymology has often associated the first element with the Scandinavian base of Old Icelandic marr sea, this is unlikely for phonological reasons. The second element is from the same Scandinavian base as Old Icelandic eldr fire.