Other forms: idee fixes
An idee fixe is a thought you can't get out of your head, no matter how hard you try. Worrying obsessively about your health — panicking at the slightest sniffle — is an example of an idee fixe.
In psychology, idee fixe is another name for a fixation, an unhealthy preoccupation with something or someone. It's completely natural to be concerned about climate change, but being unable to think or talk about anything else for weeks or months makes it an idee fixe. In fiction, you can think of Captain Ahab's obsession with killing the whale Moby Dick as his idee fixe. The French idée fixe means "fixed idea."