When you have a boat in poker (or full boat poker), you have a full house. In other words, you have three cards with the same face value and two cards with the same face value.
For example, you’re holding pocket tens, and the board comes T 2 2. You now have Tens full of Aces. Or, if you have 4 5 and the flop comes 4 4 5, you have 4s full of 5s.
This is obviously one of the strongest poker hands, so if you could, you’d always want to have a full boat when playing this game.
For example, you could also this poker term to describe a situation you don’t want to find yourself in by saying something like, “I have the best hand, but the opponent might make his boat on the river.”
This makes it one of the coolest poker slang terms, as you can use it with different metaphors so that very few players, especially unskilled ones, can hardly understand you.