The highest possible hand in poker. Playing Hold 'em, if you hold As, 3s and the board is Qs, 8s, Kd, 2s, 9c you are said to be holding "the nuts." The terminology is said to refer to the fact that poker is a partial information game, meaning that when you bet, you don't know if you're winning or losing. This being the case, you have to have "nuts" (as in testicles) to have the nerve to bet. The only case when you can bet in poker without "testicles" would be if you had the best possible hand and could not be beaten, hence the term "nuts"
Joe won that large pot over Carl because Carl only had a flush, but Joe had the nuts with four of a kind.