sentry
 oxford
 views  updated Jun 11 2018sen·try  / ˈsentrē/ •
n. (pl.  -tries) a soldier stationed to keep guard or to control access to a place.PHRASES: stand sentry keep guard or control access to a place.
 The Oxford Pocket Dictionary of Current English 
   sentry
 oxford
 views  updated May 23 2018sentry †sentinel; armed member of the fighting forces set to keep guard. XVII. perh. shortening of †
centrinell, 
-onel (XVI), vars. of 
SENTINEL, with assim. to 
-RY. comp. 
sentry-go, †orig. phr. consisting of 
sentry sb. (used vocatively) and imper. of Go; (hence) patrol or duties of a s. XIX.
  The Concise Oxford Dictionary of English Etymology T. F. HOAD