Wiktionary
heart of hearts
n. One's innermost private feelings.
Wikipedia
Heart of Hearts
Heart of Hearts was Bobby Vinton's twenty-sixth studio album and his second studio album for ABC Records. It was released in 1975. " Beer Barrel Polka" is the album's only single. Cover versions include Morris Albert's hit " Feelings" and Elvis Presley's hit " Wooden Heart".
Usage examples of "heart of hearts".
I don't look it, but I am beginning to feel it in my heart of hearts.
Somehow, in his heart of hearts, he knew now there always would be.
He worked on tiny animals, breaking their heart of hearts and recombining it in new ways.
She knew he was not Saldaean, but it had been so hard, thinking in her heart of hearts that he believed her too weak to stand up to him.