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1856, "something that pulls," earlier used of a quack device consisting of two metal rods which were supposed to relieve rheumatism (1798, in full Perkins's metallic tractor), from Medieval Latin tractor, from stem of Latin trahere "to pull, draw" (see tract (n.1)).
Sense of "an engine or vehicle for pulling wagons or plows" is recorded by 1896, from earlier traction engine (1859). The meaning "powerful truck for pulling a freight trailer" is first found 1926; tractor-trailer is attested from 1949.