Etymology
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sentry (n.)

1610s, originally "watchtower;" perhaps a shortened variant of sentinel, which had a variant form centrinel (1590s); or perhaps worn down from sanctuary, on notion of "shelter for a watchman." Meaning "military guard posted around a camp" is first attested 1630s. Sentry-box is from 1728.

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Definitions of sentry

sentry (n.)
a person employed to keep watch for some anticipated event;
Synonyms: lookout / lookout man / sentinel / watch / spotter / scout / picket
From wordnet.princeton.edu