The saying “a man of my kidney” is not now in frequent use; we are much more likely to say, “a man after my own heart,” which carries the same meaning.
But we find it used by Shakespeare and other writers of bygone years. The explanation takes us back to the philosophy of the Middle Ages, to the time when the temperament of men was supposed to be governed by their “humours.”
A person who was sullen or gloomy was supposed to have too much black bile, which gives us, from the Greek, the word “melancholy”; one who was irascible was supposed to be bilious, giving us “choler”; one of ruddy complexion was supposed to be cheerful, hopeful, and amorous, hence, full of blood, giving us “sanguine”; one of dull and sluggish nature, or cool and self-possessed, was supposed to have an excess of phlegm, giving us “phlegmatic.”
In the same philosophy, the kidney was supposed to be the seat of the affections; thus, in the original sense, “a man of my kidney” meant a person whose temperament and disposition were the same as those of the speaker.