bluecoat

noun3

1. •
   (US) A soldier or officer in the Union army during the American Civil War.

   • The shot wounded Thomas Lord, a 73-year-old former New York City police officer from Suffolk, Va., and a bluecoat with the Seventh New York Volunteer Cavalry. — 2009
2. •
   [before a noun] (UK) A pupil attending any of certain prestigious British schools having a traditional dress code.

   • This was a bluecoat school in my day.
3. •
   (old-fashioned) A policeman.

   • "The offices are up those steps," said the bluecoat. — 1900