Abarim Publications' online Biblical Hebrew Dictionary
The origin (and meaning) of the feminine noun פלדה (pelada) is a mystery. In the Bible this noun is used only once, in Nahum 2:3, where it reads: "the chariots are enveloped in flashing something".
Our noun is traditionally translated with "steel/iron" but that's not more than a guess. This word doesn't seem to exist in other languages, although some scholars have proposed it's a loan-word from Persian.