dead secret

例句：

1. 1. The dead secret of the past is still a mystery to us today.
   过去的死秘密对我们来说仍然是个谜。
2. 2. He kept the dead secret of their love for each other.
   他保守着他们之间死的秘密。
3. 3. The police were unable to solve the dead secret of the murder.
   警方无法解开这起谋杀案的死秘密。