In poker, the nut hand, or just the nuts, is the strongest hand possible in any particular situation. The term applies mostly to community card poker games to mean the individual holding that makes the strongest hand possible with the given board of community cards. By extension, the term is used more loosely to refer to any very strong hand.
For example in Texas hold ’em, if the board is 5♠ 6♠ A♣ 9♠ 5♥, a player holding 7♠ 8♠ has the nuts (a 9-high straight flush in spades), and cannot lose. Sometimes it is useful to know that your hand is the second or third best possible. On this same board, the hand 5♣ 5♦ would be the second-nut hand, four fives; and the third-nut hand would be any pair of the remaining three aces, making a full house A-A-A-5-5.
In high-low split games one often speaks of “nut low” and “nut high” hands separately. With an Omaha board identical to the one above, any hand with 2-3 makes the nut low 6-5-3-2-A, while 2-4 is the second-nut low (the nut high hands remain the same).
Finally, one also hears terms such as “nut flush” or “nut full house” to mean the highest hand possible in that particular category in the circumstances, even though that may not be strictly the nut hand. For example, a pair of aces with the above board could be called the “nut full house”, even though there are two higher (but very unlikely) hands possible.
The phrase originates from the historical poker games in the colonial west of America. If one bet to the sum of everything he possessed, he would place the “nuts” of his wagon wheels on the table. Most likely, this was to ensure that, should the wagerer lose the hand, he would be unable to flee and would have to make good on the bet. Obviously, to make such a bet one would need to be sure that he has the best possible hand.
There is also a possibility of having a nut losing hand (a hand that will lose to anything). For example, this occurs when the board has four of a kind and a deuce. In this situation, if you hold pocket 2’s, there is no possibility of this hand winning a showdown with any other hand, as any opponent must have a better kicker than you.