A device or part that joins or connects two things together, like the mechanism that connects railroad cars or the part that links a drive shaft.
From 'couple' + '-er' (one who does something). The term emerged in the 1800s as mechanical engineering developed, needing words for the hardware that physically connected components.
The automatic coupler, invented by Eli Janney in 1873, revolutionized railroads by letting workers couple cars without standing between them—it saved so many lives that it's considered one of the most important safety inventions in history.
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