E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Gall of Bitterness (The).
The bitterest grief; extreme affliction. The ancients taught that grief and joy were subject to the gall, affection to the heart, knowledge to the kidneys, anger to the bile (one of the four humours of the body), and courage or timidity to the liver. The gall of bitterness, like the heart of hearts, means the bitter centre of bitterness, as the heart of hearts means the innermost recesses of the heart or affections. In the Acts it is used to signify the sinfulness of sin, which leads to the bitterest grief.
I perceive thou art in the gall of bitterness, and in the bond of iniquity.Acts viii. 23.