'Sentry box' definitions:

Definition of 'sentry box'

noun

A small shelter with an open front to protect a sentry from the weather

From: GCIDE

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]