the depths of one’s conscience or emotions
This anatomically curious but firmly established expression is a variant of the older and more comprehensible heart of heart, meaning very centre of the heart, which was coined by the English poet and playwright William Shakespeare (1564-1616) in The Tragicall Historie of Hamlet, Prince of Denmarke (between 1599 and 1602); Hamlet says to his friend Horatio:
(Quarto 2, 1604)
Giue me that man
That is not passions slaue, and I will weare him
In my harts core, I [= ay] in my hart of hart
As I doe thee.
In The tragedy of Hamlet, Prince of Denmark as it is now acted at His Highness the Duke of York’s Theatre (published in 1676), an adaptation of Shakespeare’s play by the English poet and playwright William Davenant (1606-68), the expression has its current form:
Give me that man
That is not passions slave, and I will wear him
In my hearts core, I, in my heart of hearts
As I do thee.
A variant was heart’s heart, as in Du Bartas his deuine weekes and workes translated: and dedicated to the Kings most excellent Maiestie by Iosuah Syluester (1611), by Joshua Sylvester (1563-1618) (Guillaume de Salluste (1544-90), seigneur Du Bartas, was a Gascon Huguenot courtier and poet):
ABRAHAM. The THIRD DAY Of The SECOND WEEK:
O IACOB’s seed (I might say, my deer sons)
Y’ are sense-les more then metalls, stocks or stones,
If y’ have forgot the many-many Miracles
Wher-with the Lord hath seal’d my sacred Oracles;
And all the Favours (in this savage Place)
In forty yeers received of his grace.
Therefore (O ISRAEL) walk thou in his fear,
And in thy hearts-heart (not in Marble) bear
His ever-lasting LAVV.