A soldier, especially one in a cavalry or armored unit, or a state police officer.
From 'troop,' which comes from French 'troupe' meaning 'a group.' A trooper was originally a soldier who fought as part of a mounted troop. The word evolved to mean both cavalry soldiers and modern police officers.
The phrase 'trooper' has a special meaning in American English specifically—when someone is called 'a real trooper,' it means they handle tough situations with courage and persistence, which shows how military terminology has seeped into everyday language as a compliment.
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