Source : Webster's Revised Unabridged Dictionary (1913)

Guardsman \Guards"man\, n.; pl. {Guardsmen}.
   1. One who guards; a guard.

   2. A member, either officer or private, of any military body
      called Guards.

Source : WordNet®

guardsman
     n : a soldier who is a member of a unit called `the guard' or
         `guards'