1911 Encyclopædia Britannica/Sentinel

SENTINEL, or Sentry, a guard or watch, a soldier posted at a particular spot to challenge all corners, passing those who give a countersign, and refusing those who do not, and giving alarm in case of attack. The etymology has been the subject of much controversy. The original word seems to be Ital. sentinella, adapted as Fr. sentinelle (the modern French military term is factionnaire, and the Ger. Fachmann). For the Italian word the source has been suggested in sentire, to perceive, but there are philological objections to this, and more plausibility attaches to a connexion with sentina, the bilge-water in a ship, figuratively rabble, camp-followers. If an Italian origin, as agreed on by most authorities, be set aside, the French word suggests a more appropriate formation as the diminutive of sentier, path, Lat. semita, meaning properly the sentry’s beat. The O. Fr. senteret (a form of sentier) would account for the English form “ sentry.”