trooper

n. 1. A soldier in a body of cavalry; a cavalryman; also, the horse of a cavalryman. [ 1913 Webster ]

2. A state police officer; -- also called state trooper. [ U. S. ] [ PJC ]

3. A mounted policeman. [ Australia ] The black troopers of Queensland are a regiment of aboriginal police, employed chiefly for dispersing wild aborigines who encroach on sheep runs. [ Webster 1913 Suppl. ]

4. Trouper. [ PJC ]

like a trooper, with energy, endurance, or enthusiasm; as, to work like a trooper.