a soldier stationed at a place to stand guard and prevent the passage of unauthorized persons, watch for fires, etc., especially a sentinel stationed at a pass, gate, opening in a defense work, or the like.
1611, originally "watchtower;" perhaps a shortened variant of sentinel (q.v.), which had a variant form centrinel (1598), or worn down from sanctuary, on notion of "shelter for a watchman." Meaning "military guard posted around a camp" is first attested 1632. Sentry-box is from 1728.