The phrase to return to one’s muttons means to return to the matter in hand.

This phrase is from French revenons à nos moutons (let us return to our sheep—cf. footnote), which is said to have originally been an allusion to the line Sus ! revenons à ces moutons (Come now! let us return to these sheep), spoken by a judge seeking to bring a digressing litigant back to the question of some stolen sheep in a court scene in La Farce de Maistre Pierre Pathelin, written around 1457 and extremely popular in its day. However, the unknown author of this French farce perhaps used an already established phrase. In any case, as early as around 1480, the French poet Guillaume Coquillart (1452-1510) used revenons à nos moutons in Le Monologue de la botte de foin (The Monologue of the bundle of straw), and the French satirist François Rabelais (circa 1494-1553) employed it frequently.

It is interesting to note that the Roman epigrammatist Martial (circa 40-circa 104 AD) had used a similar image:

My suit has nothing to do with assault, or battery, or poisoning, but is about three goats, which, I complain, have been stolen by my neighbour. This the judge desires to have proved to him; but you dilate on the Battle of Cannae, the Mithridatic war, and the perjuries of the insensate Carthaginians, the Sullae, the Marii, and the Mucii, with swelling words and extravagant gestures. It is time, Postumus, to say something about my three goats.
     original text:
Non de vi neque caede nec veneno,
Sed lis est mihi de tribus capellis:
Vicini queror has abesse furto.
Hoc iudex sibi postulat probari:
Tu Cannas Mithridaticumque bellum
Et periuria Punici furoris
Et Sullas Mariosque Muciosque
Magna voce sonas manuque tota.
Iam dic, Postume, de tribus capellis.

The first known user of the English phrase was the novelist and educationist Maria Edgeworth (1768-1849), who, in a letter dated 5^th November 1820, wrote:

… How reviewers have changed since they wrote that.
But to come back to our muttons—the wind not being fair we did not sail to Dover but we are in hopes it will change before tomorrow.

As late as 1974, the New Zealand detective novelist and theatre director Edith Ngaio Marsh (1895-1982) used it in Black As He’s Painted:

The Boomer […] enlarged, with intermittent gusts of Homeric laughter, upon the machinations of the Ng’ombwanan extreme right and left who had upon several occasions made determined efforts to secure his death and were, through some mysterious process of reason, thwarted by the Boomer’s practice of exposing himself as an easy target. “They see,” he explained, “that I am not, as we used to say at Davidson’s, standing for their tedious codswallop.”
“Did we say that at Davidson’s?”
“Of course. You must remember. Constantly.”
“So be it.”
“It was a favorite expression of your own. Yes,” shouted the Boomer as Alleyn seemed inclined to demur, “always. We all picked it up from you.”
“To return, if we may, to the matter in hand.”
“All of us,” the Boomer continued nostalgically. “You set the tone at Davidson’s,” and noticing perhaps a fleeting expression of horror on Alleyn’s face, he leant forward and patted his knee. “But I digress,” he said accurately. “Shall we return to our muttons?”
“Yes,” Alleyn agreed with heartfelt relief. “Yes. Let’s.

The French phrase and its variants have occasionally been used by British persons. For instance, the English letter writer John Chamberlain (1553-1628) used pour retourner à nos moutons (to return to our sheep) in a letter which he wrote on 22^nd February 1617 to the English art collector, diplomat and Secretary of State Sir Dudley Carleton (1573-1632). And, in The History of Pendennis (1850), the English novelist William Makepeace Thackeray (1811-63) wrote:

“And so that was his brougham, sir, was it?” the nephew said, with almost a sneer.
“His brougham—O ay, yes!—and that brings me back to my point—revenons à nos moutons. Yes, begad! revenons à nos moutons. Well, that brougham is mine, if I choose, between four and seven.”

 

 

Note: In the English phrase, muttons is a mistranslation. The French masculine noun mouton denotes both the live animal and its flesh (i.e. both sheep and mutton); it is a count noun in the first case, a mass noun in the second; that is to say, it is only in the sense of live animal that the word can be plural: while un mouton means a sheep and des moutons means (some) sheep, du mouton means (some) mutton.