"four-wheeled vehicle to carry heavy loads," late 15c., from Middle Dutch wagen, waghen, from Proto-Germanic *wagna- (source also of Old English wægn, Modern English wain, Old Saxon and Old High German wagan, Old Norse vagn, Old Frisian wein, German Wagen), from PIE *wogh-no-, suffixed form of root *wegh- "to go, move, transport in a vehicle" (source also of Latin vehiculum). It is thus related to way.
In Dutch and German, it is the general word for "a wheel vehicle;" its use in English is a result of contact through Flemish immigration, Dutch trade, or the Continental wars. It has largely displaced the native cognate, wain. Spelling preference varied randomly between -g- and -gg- from mid-18c., until American English settled on the etymological wagon, while waggon remained common in Great Britain. Wagon-train is attested from 1810. Phrase on the wagon "abstaining from alcohol" is attested by 1904, originally on the water cart.
1610s, "a medium through which a drug or medicine is administered," also "any means of conveying or transmitting," from French véhicule (16c.), from Latin vehiculum "means of transport, vehicle, carriage, conveyance," from vehere "to bear, carry, convey" (from PIE root *wegh- "to go, move, transport in a vehicle," which also is the source of English wagon). Sense of "cart or other conveyance" in English first recorded 1650s.
Old English wægn "wheeled vehicle, wagon, cart," from Proto-Germanic *wagna, from PIE *wogh-no-, suffixed form of root *wegh- "to go, move, transport in a vehicle" (source also of Latin vehiculum). A doublet of wagon. Largely fallen from use by c. 1600, but kept alive by poets, who found it easier to rhyme on than wagon. As a name for the Big Dipper/Plough, it is from Old English (see Charles's Wain).
So called perhaps because the wood originally was used for wagon building and coachwork, but the sense evolution is not entirely clear. Meaning "panels lining the walls of rooms" is recorded from 1540s. Wainscoting is from 1570s.
Proto-Indo-European root meaning "to go, move, transport in a vehicle."
The root wegh-, "to convey, especially by wheeled vehicle," is found in virtually every branch of Indo-European, including now Anatolian. The root, as well as other widely represented roots such as aks- and nobh-, attests to the presence of the wheel — and vehicles using it — at the time Proto-Indo-European was spoken. [Watkins, p. 96]
It forms all or part of: always; away; convection; convey; convex; convoy; deviate; devious; envoy; evection; earwig; foy; graywacke; impervious; invective; inveigh; invoice; Norway; obviate; obvious; ochlocracy; ogee; pervious; previous; provection; quadrivium; thalweg; trivia; trivial; trivium; vector; vehemence; vehement; vehicle; vex; via; viaduct; viatic; viaticum; vogue; voyage; wacke; wag; waggish; wagon; wain; wall-eyed; wave (n.); way; wee; weigh; weight; wey; wiggle.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit vahati "carries, conveys," vahitram, vahanam "vessel, ship;" Avestan vazaiti "he leads, draws;" Greek okhos "carriage, chariot;" Latin vehere "to carry, convey," vehiculum "carriage, chariot;" Old Church Slavonic vesti "to carry, convey," vozŭ "carriage, chariot;" Russian povozka "small sled;" Lithuanian vežu, vežti "to carry, convey," važis "a small sled;" Old Irish fecht "campaign, journey," fen "carriage, cart;" Welsh gwain "carriage, cart;" Old English wegan "to carry;" Old Norse vegr, Old High German weg "way;" Middle Dutch wagen "wagon."
"ammunition wagon; wooden chest for bombs, gunpowder, etc.," 1704, from French caisson "ammunition wagon," originally "large box" (16c.), from Italian cassone, augmentative form of cassa "a chest," from Latin capsa "a box" (see case (n.2)).