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1836, U.S. colloquial, probably a fanciful construction. The "English Dialect Dictionary" reports Gallithumpians as a Dorset and Devon word from 1790s for a society of radical social reformers, and also in reference to "noisy disturbers of elections and meetings" (1770s). The U.S. reference is most commonly "a band of discordant instruments" or bangers on pots and pans, especially to "serenade" a newlywed couple to show disapproval of one or the other or the match.