trailer n.

c. 1300, "to hang down loosely and flow behind" (of a gown, sleeve, etc.), from Old French trailler "to tow; pick up the scent of a quarry," ultimately from Vulgar Latin *tragulare "to drag," from Latin tragula "dragnet, javelin thrown by a strap," probably related to trahere "to pull" (see tract (n.1)). Transitive sense of "to tow or pull along the ground" is from c. 1400. The meaning "follow the trail of" (an animal, etc.) is first recorded late 14c. Meaning "to lag behind" is from 1957. Related: Trailed; trailing.

also semitrailer, 1910 in reference to motor vehicles (late 19c. in botany), from semi- + trailer. The short form semi is attested by 1942.