The Collaborative International Dictionary of English v.0.48:

Sentry \Sen"try\, n.; pl. Sentires. [Probably from OF.
   senteret a little patch; cf. F. sentier path, and OF. sente.
   See Sentinel.]
   1. (Mil.) A soldier placed on guard; a sentinel.
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   2. Guard; watch, as by a sentinel.
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            Here toils, and death, and death's half-brother,
            sleep,
            Forms terrible to view, their sentry keep. --Dryden.
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   Sentry box, a small house or box to cover a sentinel at his
      post, and shelter him from the weather.
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