forest (n.) Look up forest at Dictionary.com
late 13c., "extensive tree-covered district," especially one set aside for royal hunting and under the protection of the king, from Old French forest "forest, wood, woodland" (Modern French forêt), probably ultimately from Late Latin/Medieval Latin forestem silvam "the outside woods," a term from the Capitularies of Charlemagne denoting "the royal forest;" perhaps via Old High German forst, from Latin foris "outside" (see foreign), with a sense of "beyond the park," the park being the main or central fenced woodland.

Another theory traces it through Medieval Latin forestis, originally "forest preserve, game preserve," from Latin forum in legal sense "court, judgment;" in other words "land subject to a ban" [Buck]. Replaced Old English wudu.
forest (v.) Look up forest at Dictionary.com
1818 (forested is attested from 1610s), from forest (n.).