Q: What does "fell" mean?
A: It means fierce or deadly. "Fell" is arguably close to being archaic. Apart from Tolkien, I know it only from the oft quoted phrase from Macbeth "One fell swoop". Which is often misquoted as "One foul swoop", precisely because of the unfamiliarity of "fell" to modern English speakers. But "fell" may yet be saved by the popularity of Tolkien - or even by the "Fell Beast" collectibles sold following the movies.