A person or vessel that contains or measures out liquids by the gallon; someone who sells by the gallon.
From 'gallon' plus '-er' (agent suffix). This occupational term emerged when gallon-based commerce became standardized.
In early America and Britain, a 'galloner' was an actual job—someone at a market stall measuring and selling milk, oil, or ale by the gallon, and getting the measurement wrong could mean real trouble.
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