It is a word within the Norse mythology but the meaning is still unknown. It was used in the japanese animation titled "Mahou Shoujo Lyrical Nanoha A's" by a newly formed mage named Yagami Hayate, who is also called Queen of the Night Sky. It is a very powerful magic spell that cause the target to turn into stone.
Approach from the beyond, mistletoe branches, and become spears of the silver moon! Pierce it! Stone spears, Misteltein!