Be onto Definition 1 of 4Pro Tip 1/3Suspicion Alert Used when someone suspects another's intentions or actions are secretive. I'm onto you; no sneaking extra cookies! Pro Tip 2/3Discovery Highlight Emphasizes realizing or uncovering something hidden or unexpected. She's definitely onto something with her new research. Pro Tip 3/3Informed Insight Indicates having insider or advanced knowledge about a situation. The journalist is onto the scandal before it hits the news.