The Collaborative International Dictionary of English v.0.48:
Sentinel \Sen"ti*nel\, n. [F. sentinelle (cf. It. sentinella);
probably originally, a litle path, the sentinel's beat,, and
a dim. of a word meaning, path; cf. F. sente path. L. semita;
and OF. sentine, sentele, senteret, diminutive words. Cf.
1. One who watches or guards; specifically (Mil.), a soldier
set to guard an army, camp, or other place, from surprise,
to observe the approach of danger, and give notice of it;
The sentinels who paced the ramparts. --Macaulay.
2. Watch; guard. [Obs.] "That princes do keep due sentinel."
3. (Zool.) A marine crab (Podophthalmus vigil) native of
the Indian Ocean, remarkable for the great length of its
eyestalks; -- called also sentinel crab.