Definition of jinx card
The jinx card is the unlucky card. An unlucky card can also be a card that gives implied 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. To us, before we see our opponent's cards, this is a good card because it gives us the flush, but in reality, it is a bad luck card because it is on this card that we will lose a huge pot.
Other meanings of jinx
In poker, we can also talk about "jinxing yourself". In other words, to bring bad luck to yourself if you are superstitious. For example, if you say "every time I have kings, my opponent has aces" a player might say "don't say that, you'll jinx yourself" in other words, don't say that, or else bad luck will strike you. These are obviously superstitions. You either believe it or you don't, but these are phrases and expressions that you will hear at the poker table.