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"to jinx it" means to make it have bad luck.

These cards are jinxed. I never win any money when I play with them.

Being jinxed generally means having bad luck. But in the context you are using it, "Jinx" is the verb or action, which means "to cause bad luck," not just a general feeling of bad luck. The phrase "Don't Jinx It" is most often used in a situation where someone proclaims that things are going well or that they think something good is about to happen to them. The superstition is that discussing your perceived good fortune will come across like you aren't appreciating it, and thus you lose it. It is also used when someone proclaims that things couldn't get any worse. The superstition here is that saying it couldn't get worse has now guaranteed it will get worse. It's meaning here is similar to the other example in the sense that no matter how poor your fortune, it could always be worse and you should realize that and cherish what you have.