E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Heart of Hearts (In one’s).
In one’s inmost conviction. The heart is often referred to as a second self. Shakespeare speaks of the “neck of the heart” (Merchant of Venice, ii. 2); “the middle of the heart” (Cymbeline, i. 7). The heart of the heart is to the same effect.
