1. To have knowledge, be suspicious, or be aware of someone's actions, behavior, or intention. You'd better tread carefully, I'm on to you now. The police are on to us! Everybody run!

2. To be in contact with someone at a given point in time. I'll be on to you tomorrow about the details of our meeting. I was on to John yesterday about where he wants to go for his birthday.

3. To discover, realize, or be in the process of doing something of great importance, value, or insight. Great work on this essay, Lindsay. I really think you're on to something here. Scientists now believe they may be on to a cure for cancer.