An idiom meaning unprecedented and unlikely to be repeated; used to emphasize something extremely remarkable or extraordinary (for better or worse).
彼の業績は空前絶後と評価された。
His achievements were described as unprecedented and unlikely to be repeated.
そのコンサートは空前絶後の盛り上がりだった。
That concert was an unprecedented, once-in-a-lifetime spectacle.
空前絶後のチャンスを逃してはいけない。
You must not miss this once-in-a-lifetime (unprecedented) opportunity.