greatest mistake of one's life
A dramatic set phrase used when someone makes an irreparable, deeply regrettable blunder that will haunt them forever. Often used in self-reproach or to describe a catastrophic error.
Making such a simple mistake is the greatest blunder of my life.
He regrets that decision as the worst mistake of his life.
不覚 alone means 'failure' or 'blunder', but 一生の不覚 specifically emphasizes it as the single worst mistake in one's entire life.
大失敗 is a general 'big failure', while 一生の不覚 carries a heavier, lifelong regret and is more dramatic.
A set phrase combining 一生 (one's whole life) and 不覚 (failure, blunder). The exact origin is uncertain, but it has been used in Japanese literature and speech to express a mistake so severe it defines one's life.