carry a secret to the grave
carry a secret to (one's)/the grave
To keep (not reveal) a secret for the duration of one's life. I can't believe that grandma carried such a huge family secret to her grave! If Uncle Joe hadn't blurted it out, we'd never have known about it! Oh, Emily is a loyal friend—she would definitely carry a secret to the grave.
carry a secret to the graveand carry a secret to one's grave
Fig. to never reveal a secret, even to the day of one's death. John carried our secret to his grave. Trust me, I will carry your secret to the grave!