terrain (n.)

1727, "ground for training horses," from French terrain "piece of earth, ground, land," from Old French (12c.), from Vulgar Latin *terranum, from Latin terrenum "land, ground," noun use of neuter of terrenus "of earth, earthly," from terra "earth, land," literally "dry land" (as opposed to "sea"); from PIE root *ters- "to dry." Meaning "tract of country, considered with regard to its natural features" first attested 1766.