What is "jinxing"

Usage examples of "jinxing".

For the past decade, a good day was one without an incident, never mind feeling happy, so she relished the gift she'd been given today and didn't really want to consider it too closely for fear of jinxing it.

Ron, who was supposed to be jinxing Harry, was purple in the face, his lips tightly compressed to save himself from the temptation of muttering the incantation.