Definition of jinx card
The jinx card is the card that brings bad luck. A jinx card can also be a card that gives implicit odds to our opponent. For example, if we have KJ in spades and our opponent has A4 in spades on 2-7-9-T-5 with three spades, the 5 of spades on the river that completes the suit will be considered the jinx card. For us, before we see our opponent's cards, it is a good card because it gives us the suit, but in reality it is a card that brings us bad luck because it is on this card that we will lose a huge pot.
The other meanings of jinx
In poker, we can also talk about "jinxer oneself". In other words, to bring bad luck to oneself if one is superstitious. For example, if you say "every time I have the kings, my opponent has the aces" a player might tell you "don't say that, you're going to jinxer yourself" in other words, don't say that, or bad luck will hit you. This is obviously superstition. You either believe it or you don't, but it's still phrases and expressions you'll hear at the poker table.