The Dialogue Concerning the Exchequer, circa 1080, Book 1, § 12
The forest of the king is the safe dwelling-place of wild beasts; not of every kind, but of the kinds that live in woods; not in all places, but in fixed ones, and ones suitable for the purpose; whence it is called "forests," the " e " being changed into " o," as if it were " feresta " . i.e., an abiding place for wild beasts.