Likely from the mithril mail coat that Bilbo and Frodo wear in The Hobbit and Lord of the Rings. Note that that coat was made by dwarves, for an elf, so it's really dwarf mail.
With that he put on Bilbo a small coat of mail, wrought for some young elf-prince long ago. It was of silver-steel which the elves call mithril, and with it went a belt of pearls and crystals. A light helm of figured leather, strengthened beneath with hoops of steel, and studded about the bring with white gems, was set upon the hobbit's head.
JRR Tolkien, The Hobbit