on·to / ˈänˌtoō; ˈôn-/ • prep. 1. moving to a location on (the surface of something): they went up onto the ridge.2. moving aboard (a public conveyance) with the intention of traveling in it: we got onto the train.PHRASES: be onto someone inf. be close to discovering the truth about an illegal or undesirable activity that someone is engaging in.be onto something inf. have an idea or information that is likely to lead to an important discovery.