The phrase cock-and-bull story denotes an implausible story used as an explanation or excuse.

The French expression sauter du coq à l’âne, literally to jump from the cock to the (male) ass, means to skip from one subject to another, the image being that incoherent talking is like switching abruptly from the topic of the cock to that of the ass. The following is from A Dictionarie of the French and English Tongues (1611), by Randle Cotgrave (asne used to be a common spelling of âne):

Sauter du coq à l’asne. To run, without order, out of one matter into another.

The original phrase was saillir du coq en l’asne, to spring from the cock to the (male) ass¹, first recorded in Respit de la Mort, a poem written in 1376 by Jean Le Fèvre de Ressons (circa 1325-circa 1380). About his own text, the poet remarks:

Tant ay sailli du cocq en l’asne
Et ay divers chemins tenu
Que je suy jusquez chy venu.
     translation:
So much have I sprung from the cock to the ass
And have divers paths taken
That I have up to here come.

¹ But, in La Puce à l’oreille. Anthologie des expressions populaires avec leur origine (1985 edition), the French author Claude Duneton (1935-2012) suggests a different primary signification. According to him, in the original phrase, the verb saillir had its other meaning, to cover, that is, to copulate with, and asne was not the noun designating an ass but the obsolete noun (also spelt ane) denoting a female duck. (The nouns âne and asne (or ane) are respectively from Latin asinus and anas.)

As a noun, un coq-à-l’âne is an abrupt change of subject. This noun also denotes a satirical and burlesque epistle² consisting in a voluntarily incoherent discourse and unbridled imagination. The poet of the Renaissance period Clément Marot (1496-1544) is credited with inventing this type of epistles.

² An epistle is a poem or other literary work in the form of a letter or series of letters.

In the early 17^th century, Scottish borrowed coq-à-l’âne as cockalane, in the senses of a satire, lampoon and of a disconnected story or discourse (in the above-mentioned dictionary, Randle Cotgrave translated the noun “coq-à-l’asne” as “a libell, pasquin, satyre”).

The English expression also appeared in the early 17^th century. Its original forms were to talk of a cock and a bull, meaning to tell a long rambling, idle story, and a story of a cock and a bull, meaning tedious, disconnected or misleading talk.

It is first attested in The Anatomy of Melancholy. What it is, with all the kinds causes, symptomes, prognostickes, & seuerall cures of it (first published in 1621), written by the English scholar Robert Burton (1577-1640) under the pen name of Democritus Junior:

(1628 edition)
Some mens sole delight is, to take Tobacco, & drinke all day long in a Tauerne or Ale-house, to discourse, sing, iest, roare, talke of a Cock and a Bull ouer a pot &c.

Interestingly, in The Christian’s Exercise: or Rules to live above the world while we are in it (first published in 1715), the translation by Robert Nelson of a book written in German by the canon regular Thomas à Kempis, the formulation very much resembles the French expression:

(1717 edition)
Neither their Words nor their Actions have any regular Connexion; nor do they observe any Order at all; there being here neither Beginning nor End, or, as they commonly say, ‘Neither Head nor Tail’; but they skip from a Cock to a Bull: For they seem to be in a Dream with their Eyes open.

Indeed, it is quite likely that the French expression was borrowed in partial translation, with ass changed to bull for an unknown reason. This seems to be supported by the Scottish borrowing from French. It is therefore likely that in the English expression too, the image is that disjointed talk is like jumping from the topic of a cock to that of a bull.

A popular, but erroneous, theory is that the English phrase was originally referring to an early fable — now conveniently lost — evoking a cock and a bull metamorphosed into a single animal.

But this theory is based on a single text, written several decades after the first attestation of the phrase. Additionally, the relation between this text, which illustrates an “illogical way of Argumentation”, and the phrase is unclear. It is most likely that this relation is coincidental, or that the author was punning on the existing phrase.

The text in question is from Rusticus ad academicos in exercitationibus expostulatoriis, apologeticis quatuor. The rustick’s alarm to the rabbies, or, The country correcting the university and clergy, and (not without good cause) contesting for the truth, against the nursing-mothers, and their children (1660), by the Quaker preacher and writer Samuel Fisher (1604-65); incidentally, the author does not evoke “an early fable”:

What a strange story is here, as if a man should tell a tale of two things, a Cock and a Bull, metamorphos’d into one, whereof the one having been as confidently, as untruly avowed to be assuredly known to be the other, viz. The Cock to be a Bull, is [being denied] as Ridiculously, as Reasonlesly profer’d to be proved in this illegal, and illogical way of Argumentation, viz. That which evidenceth it self to be a Bull, both is, and is assuredly known to be a Bull; but the Bull [alias, the Cock, for so he means, & should say] evidenceth himself to be a Bull: Therefore the Bull [or the Cock] both is, and is assuredly known to be a Bull.