Definition from Wiktionary, the free dictionary
- (literally) A rat that lives on or along a river.
- (chiefly pejorative) A lower-class person living on or along a river.
- (chiefly pejorative) A poker player that wins a Texas Hold'em hand by making the winning hand on the river card.
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- 1989, Orson Scott Card, Prentice Alvin 
- The river rat he fought was one of a gang of them lolling under the eaves of the porthouse, probably waiting for a gaming house to open.
- 1993, Peter Filichia, Let’s Put on a Musical! 
- Cap’n Andy claims that his show boat is “one big happy family”—and that includes his wife, daughter Magnolia, as well as stars Julie and husband Steve, and black help Queenie and Joe. But it won’t be for long; because a river rat’s love for Julie goes unrequited, and he informs the sheriff that she’s a “half-breed” with Negro blood.
- 1994, Betty Bryant, Here Comes the Showboat! 
- While other children were learning how to walk, I was learning how to swim, and I knew how to set a trotline, gig a frog, catch a crawfish, and strip the mud vein out of a carp by the time I was four.
- Dad called me a river rat.
- 2000, Elvira Woodruff, The Christmas Doll 
- To her horror, she discovered that the rope she was holding was not a rope at all, but a tail. And attached to the tail was a large river rat that scrambled frantically in midair, thrashing to get away.