The name of Tol Sirion after it was inhabited by Sauron and his minions, translated “Isle of Werewolves” (S/156), a combination of tol(l) “island”, the plural in of the definite article i and the class-plural of gaur “werewolf” using the suffix hoth “host” (SA/gaur, hoth).
Conceptual Development: In Silmarillion drafts from the 1930s, this name was Tol-na-Gaurhoth (SM/311, LR/284).
toll (“island”) + in (pl. gen. article) + gaur (“werewolf”) + hoth (class plural suffix) #[HKF] with collective plurals ending in -hoth, the article can be optionally singular, cf. Naur dan i ngaurhoth.