SENTINEL, or SENTRY, a guard or watch, a soldier posted at a particular spot to challenge all comers, 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 senlina, 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 senlier, path, Lat. semita, meaning properly the sentry's beat. The O. Fr. senterel (a form of sender) would account for the English form " sentry."
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)