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.
      [1913 Webster]

   2. Guard; watch, as by a sentinel.
      [1913 Webster]

            Here toils, and death, and death's half-brother,
            sleep,
            Forms terrible to view, their sentry keep. --Dryden.
      [1913 Webster]

   Sentry box, a small house or box to cover a sentinel at his
      post, and shelter him from the weather.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sentry box
    n 1: a small shelter with an open front to protect a sentry from
         the weather

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "sentry box":
   Nissen hut, Quonset hut, booth, crib, gatehouse, hut, hutch, kiosk,
   lean-to, outbuilding, outhouse, pavilion, shack, shanty, shed,
   stall, tollbooth, tollhouse