What does it mean by "not on my watch?"

Native speaker’s answer
Rebecca
Good question! It might be better to explain the origins of this expression first. "Watch" originally referred to one's military or naval sense of duty. For a soldier or officer to "be on watch" meant to be on duty and therefore responsible for others/things during that period of time. Eventually this expression was used outside of the military context and now it's simply an exaggerated way of saying "no way" or "not while I am around". Here, Ben 10 says "not on my watch" to express that he is suspicious of the villains and won't allow anything bad to happen while he is there. Ex: Nothing bad will happen! Not on my watch. Ex: You think someone will cause problems? Not on my watch.