α. 1800s– thoroughbrace.
β. 1800s– throughbrace Brit. 
/ˈθruːbreɪs/, U.S. /ˈθruˌbreɪs/ (rare).
thoroughbracen.
/ˈθʌrəbreɪs/, U.S. /ˈθəroʊˌbreɪs/, /ˈθərəˌbreɪs/ Originally and chiefly U.S. Now historical.
1. Any of a number of strong leather straps supporting the body of a coach or similar vehicle.
ΘΚΠ
society > travel > means of travel > a conveyance > vehicle > cart, carriage, or wagon > parts of cart or carriage > [noun] > braces supporting body
main-brace1680
brace of a coach1745
thoroughbrace1801
1801 National Intelligencer & Washington Advertiser 25 Sept. Elastic thorough braces..will entirely save the expence of common steel springs.
1837 H. Martineau Society in Amer. II. 175 Half a mile before reaching the place..the thorough-brace broke, and we had to walk..to the inn.
1858 O. W. Holmes Deacon's Masterpiece 22 In building of chaises..There is always somewhere a weakest spot,—In hub, tire, felloe, in spring or thill,..In screw, bolt, thoroughbrace.
1878 Deb. House of Commons (Canada) 25 Feb. 524/1 The only means of travelling through their mountainous country was in through brace coaches, with strong and safe brakes.
1924 Youth's Compan. 23 Oct. 696/1 The stage swung like a crazy rocking chair on its thorough-braces.
1996 H. Parker Herman Melville I. ii. 32 The phaeton was titanic, with..the body attached to the gear by throughbraces.
2013 K. Shepard Celestials xiii. 240 Its paint was chipped; its leather thoroughbraces, trunk, and driver's boot as cracked as a creek bed during a drought.
2. A vehicle whose body is supported on thoroughbraces, esp. a stagecoach.
ΘΚΠ
society > travel > means of travel > a conveyance > vehicle > cart, carriage, or wagon > [noun] > supported on thoroughbraces
thoroughbrace1878
1878 Ukiah (Calif.) City Press 9 Aug. John..was on hand with his four-horse thorough-brace, and in a few minutes..we were flying over the road.
1886 Leslie's Pop. Monthly Dec. 722/1 The mustangs looked worse than the thorough-brace itself.
1930 A. Groom Merry Christmas xv. 111 The heavily laden thoroughbrace was hitched behind seven lively horses.
1986 Sunday Mail (Queensland, Austral.) (Nexis) 22 June There is one pretty little wagon called a thoroughbrace, worth at least $6000.
Derivatives
ˈthoroughbraced adj. (of a vehicle) having a body suspended by thoroughbraces (also of the body itself).
ΘΚΠ
society > travel > means of travel > a conveyance > vehicle > cart, carriage, or wagon > [adjective] > with specific fittings
bailed1548
crane-necked1699
fallback1760
thoroughbraced1858
limbered1920
1858 N.Y. Herald 11 Nov. 1/2 Pope's new road..was full of stumps and bunches of weeds, which made it by no means pleasant riding in the thorough braced wagons.
1865 Harper's Mag. Nov. 700/1 Preference to be given to a thorough-braced ambulance of Concord manufacture.
1884 S. O. Jewett Country Doctor 19 The old-fashioned thorough-braced wagon.
1922 Cases Court of Claims U.S. 56 248 Substituting through-braced stage wagons for the spring wagons.
1959 Daily Oklahoman (Oklahoma City) 6 Sept. (Orbit section) 16/1 Sometimes he took Luz for a whirl around the estate in baronial style, seated in a shiny thoroughbraced coach drawn by six matched bays.
1994 A. C. Greene 900 Miles on Butterfield Trail i. 18 The driver's seat..was also attached to the thoroughbraced body, allowing him an easier ride.
This entry has been updated (OED Third Edition, June 2017; most recently modified version published online March 2022).
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n.1801