Bowels of Mercy
Compassion, sympathy. The affections were at one time supposed to be the outcome of certain secretions or organs, as the bile, the kidneys, the heart, the head, the liver, the bowels, the spleen, and so on. Hence such words and phrases as melancholy (black bile); the Psalmist says that his reins, or kidneys, instructed him (Psa. x. 7), meaning his inward conviction; the head is the seat of understanding; the heart of affection and memory (hence “learning by heart”), the bowels of mercy, the spleen of passion or anger, etc.
“Joseph made haste, for his bowels did yearn upon his brother.” - Gen. xliii. 30; see also 1 Kings iii. 26.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894