jinx

A hex; an evil spell.

A person or thing supposed to bring bad luck.

To cast a spell on.

To bring bad luck to.

To cause something to happen by mentioning it, usually sarcastically.

“So you'll all be near New York!” Maggie says. “We don't know for sure yet.” Sara stresses. “Don't jinx it.”

I've no idea if she guessed what I was intending to do. I don't know why I was so reluctant to talk about it, even to her. Maybe I was afraid that verbalising my intentions would jinx it in some way.

Used after the same thing is said by two people simultaneously.

Bart: I'm telling Mom and Dad! / Lisa: You're telling who? / Bart: Mom and Dad! / Girls: MOM AND DAD? JINX! / Janey Powell: Now you can't talk, 'til somebody says your name!