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’
    • ‘But her blissful countenance made every bad thing 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.’
    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/