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 .