"Don't have a fit about it!" someone might snap at you. Chances are you're making a fuss and acting out, characteristics that precisely fit or match what having a fit involves. Chill out.
When something fits like a glove, it fits just right. When you put a glove on, you fit each finger into its place. The word fit can mean "matching the shape of something" or "being a good match," like two people who meet online and turn out to be a good fit for one another. But its definitions are not always so rosy. If you're really angry and find yourself throwing a tantrum, you're having a fit.