bandwagon n.

"an organized group," originally especially of armed men, late 15c., from French bande, which is traceable to the Proto-Germanic root of band (n.1), perhaps via a band of cloth worn as a mark of identification by a group of soldiers or others (compare Gothic bandwa "a sign"). But perhaps it is rather from Middle English band, bond in the sense "force that unites, bond, tie" (late 14c.). Also compare Old Norse band "cord that binds; act of binding," also "confederacy."

The extension to "group of musicians" is c. 1660, originally musicians attached to a regiment of the army and playing instruments which may be used while marching. To beat the band (1897) is to make enough noise to drown it out, hence to exceed everything. One-man band is recorded by 1931 in the sense of "man who plays several musical instruments simultaneously;" the figurative extension is attested by 1938.

"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.