Definition of be on to someone in English:

be on to someone

phrase

informal
  • Be close to discovering the truth about an illegal or undesirable activity that someone is engaging in.

    • ‘The cops were onto us and there was absolutely nothing I could do about it.’
    • ‘What he most feared, however, was that Elsa was onto him.’
    • ‘They thought they were onto somebody who was robbing the house.’
    • ‘More importantly, how did Renee know she was onto her?’
    • ‘She must have realized he was onto her, for then she relaxed herself and allowed her soft pink lips to straighten out, forming a grim line on her face.’
    • ‘He must have sensed that she was onto him, because every time she would try and approach him, when they were camped for the night, he would just look at her and get up and walk away.’
    • ‘I didn't want her to even guess that I was onto her.’
    • ‘When Jackson realized the family was onto him, he tried to send them to South America.’
    • ‘She'd been contemplating whether or not to let Brandon know she was onto him.’
    • ‘He figured Uncle Byron must have known that Devlin was onto him.’