A large decorated wagon with a band that leads a parade; now used to mean joining something popular just because everyone else is doing it.
From 'band' (a musical group) and 'wagon.' In the 1800s, political campaigns would literally parade a decorated wagon with a band around town. The phrase 'jump on the bandwagon' meant joining a popular political movement.
Politicians literally drove around in wagons with brass bands playing to get people excited about voting for them—and if you joined in, you were 'on the bandwagon,' which is why now 'jumping on the bandwagon' means just following whatever's popular!
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