The word kidney, which is attested around 1325, is of unclear origin. The second element of the Middle-English form kidenei, plural kideneiren, is apparently ey, plural eyren, meaning egg (cf. German Eier, literally eggs, used to mean testicles). The first element remains uncertain; it is perhaps identical with cud. The Anglo-Saxon name for kidney was cropp.

The word kidney, like heart and liver, was used to mean temperament (cf. also in petto). This sense is first recorded in The Merry Wives of Windsor (around 1597), by the English poet and playwright William Shakespeare (1564-1616). Mistress Ford and Mistress Page have tricked Falstaff into hiding in a laundry basket full of filthy, smelly clothes awaiting laundering because of the imminent arrival of Ford, who suspects his wife of cheating. The wives have the basket taken away and the contents thrown into the Thames. At the Garter Inn, Falstaff tells of the fear and horror he experienced:

(Folio 1, 1623)
Then to be stopt in like a strong distillation with stink-
ing Cloathes, that fretted in their owne grease:
thinke of that, a man of my Kidney; thinke of that,
that am as subiect to heate as butter; a man of conti-
nuall dissolution, and thaw: it was a miracle to scape
suffocation. And in the height of this Bath (when I
was more then halfe stew’d in grease (like a Dutch-
dish) to be throwne into the Thames, and
coold, glowing-hot, in that serge like a Horse-
shoo; thinke of that; hissing hot: thinke of that.

The word came to also mean sort, class, kind. The following is from A New Dictionary of the Terms Ancient and Modern of the Canting Crew (1699), by “B. E. Gent.”:

Of that Kidney, of such a Stamp. Of a strange Kidney, of an odd or unaccountable Humor.

In The Tatler; or Lucubrations of Isaac Bickerstaff, Esq of 28^th September 1710, the Irish satirist, poet and Anglican cleric Jonathan Swift (1667-1745) criticised the figurative use of kidney as part of “the continual corruption of our English Tongue” produced by “two evils, ignorance, and want of taste”. Instead of giving a list of “the late refinements crept into our Language”, he sent the copy of a letter that he had supposedly received, which “is in every point an admirable pattern of the present polite way of writing”. One of the “refinements” he denounced was “the choice of certain words invented by some pretty Fellows, such as “Banter, Bamboozle, Country Put, and Kidney,” as it is there applied”, referring to the following in the letter:

The Jacks and others of that Kidney are very uppish.

Similarly, in A Dictionary of the English Language (1755), by the English lexicographer Samuel Johnson (1709-84), the figurative meaning of kidney is:

Race; kind: in ludicrous language.