The English saying “to return to one’s muttons” is a direct translation from the French, revenon a nos moutons.
The literal meaning in the French phrase is the same as the English, and the figurative meanings are also the same, to return to the subject under discussion or consideration.
For the origin we must go to a sixteenth century play, Pierre Pathelin, written by the French poet, Pierre Blanchet. Pathelin (often spelled Patelin) is a lawyer who has, through flattery, hoodwinked Joceaume, the local draper, into giving him six ells of cloth.
While this injury is still rankling, Joceaume also discovers that his shepherd has stolen some of his sheep. He has the shepherd haled before the magistrate and there finds to his amazement that the shepherd has the rascally Pathelin as his lawyer.
The draper, sputtering in indignation, tries to tell the magistrate about his loss of the sheep, but each time that he sees Pathelin he begins to rave about the cloth of which he has been defrauded.
The judge begins to get somewhat confused, but tries to keep Joceaume to his charges against the shepherd: “Revenon a nos moutons (Let us return to our sheep),” he repeats time and again.