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.

    • ‘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 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.’
    • ‘He figured Uncle Byron must have known that Devlin was onto him.’
    • ‘They thought they were onto somebody who was robbing the house.’
    • ‘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.’
    • ‘The cops were onto us and there was absolutely nothing I could do about it.’
    • ‘I didn't want her to even guess that I was onto her.’
    • ‘More importantly, how did Renee know she was onto her?’
    • ‘What he most feared, however, was that Elsa was onto him.’