Terrain
early 18th century (denoting part of the training ground in a riding school): from French, from a popular Latin variant of Latin terrenum, neuter of terrenus (see terrene).
wiktionary
Borrowed from French terrain, from Latin terrenum(“land, ground”), neuter of terrenus(“consisting of earth”), from terra(“earth”).
etymonline
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.
