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." This word comes to Medieval Latin, perhaps via a Germanic source akin to Old High German forst, from Latin foris "outside" (see foreign). If so, the sense is "beyond the park," the park (Latin parcus; see park (n.)) 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 (see wood (n.)). Spanish and Portuguese floresta have been influenced by flor "flower."