be onto (one)

1. To have knowledge, be suspicious, or be aware of one's actions, behavior, or intention. You'd better tread carefully—I'm onto you now. The police are onto us! Everybody run! The FBI is onto us now? Oh man, we're doomed!

2. To be in contact with one at a given point in time. I'll be onto you tomorrow about the details of our meeting. I was onto John yesterday about where he wants to go for his birthday. Look, I was onto Grandma earlier, and she really doesn't want to do anything big for her birthday.

be ˈonto somebody

1 (informal) know about what somebody has done wrong: She knew the police would be onto them.
2 (also get ˈonto somebody) be talking to somebody, usually in order to ask or tell them something: They’ve been onto me for ages to get a job. ♢ I must get onto the local council about all the rubbish in the street.

be onto somebody

be onto (one)

be ˈonto somebody