1. A rat that lives on or along a river.
2. A lower-class person living on or along a river.
3. A poker player that wins a Texas Hold'em hand by making the winning hand on the river card.

Quotations

• 1989, Orson Scott Card, Prentice Alvin http://books.google.com/books?ie=UTF-8&hl=en&vid=ISBN0812502124&id=u8x5Q2IHw_8C&pg=PA179&lpg=PA179&sig=frn2Tg8bgUQjDpkNJJxvJOcUm9c

  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! http://books.google.com/books?ie=UTF-8&hl=en&vid=ISBN0823088170&id=-d20I7gBCs8C&pg=PA23&lpg=PA23&sig=UvFcIJYiS-CwLdeKLlKGo7fT2Ck

  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! http://books.google.com/books?vid=ISBN081311862X&id=RD-qvwUlYH0C&pg=PA1&lpg=PA1&sig=maP0RIjIHNnAVNzJOYGW01vfudw

  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 http://books.google.com/books?ie=UTF-8&hl=en&vid=ISBN0590318799&id=lHd3LzrLmDcC&pg=PA49&lpg=PA49&sig=Db_PVv86h_T9YfO5JyIsw6XRaTM

  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.