If you are overzealous, you care about something so strongly that you go too far. A zealous person is already full of heat for a cause; the over- says the heat has spilled over. An overzealous official might follow every tiny rule and make a line miserable. An overzealous fan might push too hard. The word is almost always a criticism — though actually people sometimes say it with a small smile, as if the person means well. It is a grown-up, slightly formal word. You will hear it in the news and at work more than among children.