A connection point in the street connected to the water main, used by firefighters to connect hoses and supply water for fighting fires.
From 'fire' and 'plug' (meaning a connection device, possibly from Dutch 'plug'). American term that emerged as municipal water systems developed in the 19th century.
Firefighters used to call hydrants 'fireplugs' because they literally plugged into the main water line—now the term is mostly historical, though some regions still use it!
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