Berserk comes from Old Norse berserkr, which combines ber- ("bear") and serkr ("shirt"). According to Norse legend, berserkrs were warriors who wore bearskin coverings and worked themselves into such frenzies during combat that they became immune to the effects of steel and fire. Berserk was borrowed into English (first as a noun and later as an adjective) in the 19th century, when interest in Scandinavian myth and history was high. It was considered a slang term at first, but it has since gained broader acceptance.