Definition of know something for a fact in US English:

know something for a fact

phrase

  • Be aware of something that is irrefutable or beyond doubt.

    ‘I know for a fact that he can't speak a word of Japanese’
    • ‘I know for a fact that his mother and father didn't have any siblings, so he doesn't have any cousins.’
    • ‘If you know the answer for a fact, please get in touch.’
    • ‘In an interview for BBC radio a couple of years ago, he told the interviewer that he'd never logged on to the Internet, but I know for a fact he checks his email.’
    • ‘She knew for a fact her sister had never loved William.’
    • ‘I've heard it said many times, and been long enough on this earth to know it for a fact, that marriage, as a rule, changes women more than men.’