Definition of swear up and down in US English:

swear up and down

(British swear blind)

phrase

informal
  • Affirm something emphatically.

    ‘he swore up and down they'd never get him up on that stage’
    • ‘The fortune teller swore up and down on her husband's grave that the star had not been there the night before.’
    • ‘From personal experience, every single one of my male and female friends who swore up and down that they'd never have children is now raising at least one kid, if not two.’
    • ‘More than that, have I dropped it in favour of working in London, which I swore blind I would never do while I was still at university.’
    • ‘In fact, she swore up and down that she didn't have anyone in mind, but Jane knew her better than that.’
    • ‘Naomi, as you'll know equally well, swore blind that unlike some models one wouldn't care to mention, she didn't take drugs.’
    • ‘Tony is a huge Patriots fan, a guy who swore up and down at the Super Bowl in New Orleans a few years back that New England was going to shock the world.’
    • ‘It falls into the category of ‘edgy’ family movies, where the kids swore up and down and some violence was to be had.’
    • ‘He swore up and down that it was all necessary for controling ones power but Scarlet knew he was just trying to get payback.’
    • ‘She swore up and down that she never would come here, that it was too much of a mess for her to drive all the way from Queens and back in midday traffic.’
    • ‘He told me all about it, when he was finished, and swore blind he was never going to mess with mysterious strangers again.’