In poker, the term monster refers to a very strong hand that’s likely to win the pot. It doesn’t necessarily have to be the best possible hand, but a monster hand will win a large percentage of the time.
For example, you could hear a player say, I 3-bet with Ah5h, and I flopped a monster when the flop came king-high with all hearts.
At this point, the player has the nuts, but this is a monster hand, even if the board changes and it has great potential to win a big pot against smaller flushes.
What constitutes a monster in poker changes depending on the board texture and circumstances of the hand.