One of the mysteries of the English language finally explained.
1A rat or any of various other rodents living on or near rivers.
2North American slang. A person engaging in criminal activities on a river; a river thief or river pirate. Now chiefly historical.
3North American slang. A person who works lives or works on a river.
4Specifically = river-driver. Compare river hog. Now rare.
Early 18th century; earliest use found in Aaron Hill (1685–1750), writer and entrepreneur. From river + rat.
river rat/ˈrɪvə rat/
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