Definition of I promise (or I promise you) in US English:

I promise (or I promise you)

phrase

informal
  • Used for emphasis, especially so as to reassure, encourage, or threaten someone.

    ‘oh, I'm not joking, I promise you’
    • ‘The exercise part is so simple, I promise you, you'll want to do it everyday, and I've never put that in print before.’
    • ‘I'm not one of those apathetic stay-at-home voters that the Labour Party is so worried about, I promise you.’
    • ‘Now it may seem selfish, but once you have holidayed in this stunningly beautiful country, I promise you'll be loath to share its secrets with anyone.’
    • ‘No, there will be no risk of me getting in the way, I promise you.’
    • ‘So I promise you, you're not going to hear any naughty words from him.’
    • ‘This is not an appeal to your green listeners, Amy, I promise you.’
    • ‘No, I promise it's not the makings of a classical joke, as my postman can testify.’
    • ‘Not the best body language to display, I promise you.’
    • ‘You'll never watch Jeremy Paxman's on-screen performances in quite the same way again, I promise you.’
    • ‘If you want to threaten me, I promise you that no member of your local will work here for the next 60 years.’