A soldier in a body of cavalry; a cavalryman; also, the horse of a cavalryman. [1913 Webster]
A state police officer; -- also called state trooper. [PJC]
A mounted policeman.
"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.]
Trouper. [PJC]

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

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OXFORD DICTIONARY

trooper, n.
1 a private soldier in a cavalry or armoured unit.
2 Austral. & US a mounted or motor-borne policeman.
3 a cavalry horse.
4 esp. Brit. a troop-ship.

Idiom

swear like a trooper swear extensively or forcefully.

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THESAURUS

trooper

Cheka, FBI, G-man, Gestapo, Interpol, MP, MVD, Mounties, NKVD, OGPU, RCMP, SP, Scotland Yard, bailiff, beadle, beagle, bound bailiff, captain, catchpole, cavalryman, charger, chief of police, commissioner, constable, constabulary, cossack, county police, courser, cuirassier, deputy, deputy sheriff, detective, dragoon, fed, federal, flic, gendarme, government man, heavy dragoon, highway patrol, hussar, inspector, lance, lancer, law enforcement agency, lictor, lieutenant, mace-bearer, marshal, military police, mounted policeman, narc, officer, patrolman, peace officer, police, police captain, police commissioner, police constable, police force, police inspector, police matron, police officer, police sergeant, policeman, policewoman, political police, portreeve, posse, provincial police, reeve, riot police, roundsman, secret police, security force, sergeant, sergeant at arms, sheriff, shore patrol, spahi, special police, state police, superintendent, tactical police, tipstaff, tipstaves, troopers, uhlan, vigilance committee, vigilantes, war-horse

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ROGET THESAURUS

trooper

Combatant

N combatant, disputant, controversialist, polemic, litigant, belligerent, competitor, rival, corrival, fighter, assailant, champion, Paladin, mosstrooper, swashbuckler fire eater, duelist, bully, bludgeon man, rough, prize fighter, pugilist, boxer, bruiser, the fancy, gladiator, athlete, wrestler, fighting-cock, game-cock, warrior, soldier, fighting man, Amazon, man at arms, armigerent, campaigner, veteran, swordsman, sabreur, redcoat, military man, Rajput, armed force, troops, soldiery, military forces, sabaoth, the army, standing army, regulars, the line, troops of the line, militia, yeomanry, volunteers, trainband, fencible, auxiliary, bersagliere, brave, garde-nationale, garde-royale, minuteman, auxiliary forces, reserve forces, reserves, posse comitatus, national guard, gendarme, beefeater, guards, guardsman, yeomen of the guard, life guards, household troops, janissary, myrmidon, Mama, Mameluke, spahee, spahi, Cossack, Croat, Pandoz, irregular, guerilla, partisan, condottiere, franctireur, tirailleur, bashi-bazouk, vietminh, vietcong, shining path, contras, huk, hukbalahap, mercenary, soldier of fortune, hired gun, gunfighter, gunslinger, bushwhacker, free lance, companion, Hessian, hit man torpedo, soldier, levy, draught, Landwehr, Landsturm, conscript, recruit, cadet, raw levies, infantry, infantryman, private, private soldier, foot soldier, Tommy Atkins, rank and file, peon, trooper, sepoy, legionnaire, legionary, cannon fodder, food for powder, officer, subaltern, ensign, standard bearer, spearman, pikeman, spear bearer, halberdier, lancer, musketeer, carabineer, rifleman, jager, sharpshooter, yager, skirmisher, grenadier, fusileer, archer, bowman, horse and foot, horse soldier, cavalry, horse, artillery, horse artillery, light horse, voltigeur, uhlan, mounted rifles, dragoon, hussar, light dragoon, heavy dragoon, heavy, cuirassier, Foot Guards, Horse Guards, gunner, cannoneer, bombardier, artilleryman, matross, sapper, sapper and miner, engineer, light infantry, rifles, chasseur, zouave, military train, coolie, army, corps d'armee, host, division, battalia, column, wing, detachment, garrison, flying column, brigade, regiment, corps, battalion, sotnia, squadron, company, platoon, battery, subdivision, section, squad, piquet, picket, guard, rank, file, legion, phalanx, cohort, cloud of skirmishers, war horse, charger, destrier, marine, man-of-war's man, navy, wooden walls, naval forces, fleet, flotilla, armada, squadron, man-of-war, destroyer, submarine, minesweeper, torpedo-boat, torpedo-destroyer, patrol torpedo boat, PT boat, torpedo- catcher, war castle, H, M, S, battleship, battle wagon, dreadnought, line of battle ship, ship of the line, aircraft carrier, carrier, flattop, helicopter carrier, missile platform, missile boat, ironclad, turret ship, ram, monitor, floating battery, first- rate, frigate, sloop of war, corvette, gunboat, bomb vessel, flagship, guard ship, cruiser, armored cruiser, protected cruiser, privateer, tender, store ship, troop ship, transport, catamaran, merchant marine.

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