"loud warning horn," 1908, originally on automobiles, said to have been named for the company that sold them (The Klaxon Company; distributor for Lovell-McConnell Manufacturing Co., Newark, New Jersey), but probably the company was named for the horn, from a made-up word likely based on Greek klazein "to roar," which is cognate with Latin clangere "to resound" (compare clang).
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