In the Texas Hold 'Em Poker game, when a person wins the hand in the final round and on the final community card, which is called the river. Oftentimes this person will have been losing the entire hand but gambled and got lucky on the last hand. Other players at the table will often curse this person and tag him/her a "river whore"
Frederick was winning a poker hand with three of a kind Aces, but Herman got a lucky draw on the river and wound up with a straight. Frederick verbally abused Herman and called him a river whore, while Herman snickered and collected the pot.