Definition of unhappen in US English:

unhappen

verb

[no object]
  • 1(of an occurrence) become as though never having happened; be reversed.

    ‘things had happened that could never unhappen’
    • ‘To paraphrase P.J. O'Rourke, unapproved interactions unhappen in Pyongyang.’
    • ‘The truth of the proposition is based on the fact that there has been a case of my going to Athens, and - whatever happens from now on - this cannot unhappen.’
    • ‘But her blissful countenance made every bad thing unhappen.’
    1. 1.1with object Cause (something) not to have happened.
      ‘you can't unhappen it just by saying we won't speak of it again’
      • ‘I went to see a lovely Irish padre, who said: ‘If you can't unhappen it, don't worry about it.’’

Pronunciation

unhappen

/ˌənˈhapən/