A river nigger is a Mexican as they use the river to cross over to the USA and jump fences or cross at openings in fences to gain access to the USA. A fence jumper, if you will.
Donald Trump is building a wall to keep the river niggers outta the states.
An expression used in the card game "Texas Hold 'em Poker" for someone who was losing the hand to one or more players, and was saved by the 5th card, the river, due only to luck, and won the hand.
Bob: "haha, I'm beating you with the odds of 4:1 in favour of me, you only have 3 cards you can hit out of the deck, with one card left, what a crappy call and a horrible read"
Stew: *hits the '3' he needed on the river card*
Bob: "OH MY GOD, HOW LUCKY CAN YOU GET? GO PLAY SLOT MACHINES YOU RIVER NIGGER!"
It is said of the situation where a person has the bad luck to make contact with his testicles against an undefined surface or object, intentioned or not.
Given the nature of the word, it is more appropriate to design cases where the interaction is made with a moving object, for example, a ball.
Although it is extremely painful for the victim, it tends to be considerably funny to people who witness it.
Today in the baseball game the pitcher took a nutshot; the baseball hit him in the nuts.
Man, I just watched the funniest nutshot video ever.