What do roosters have in common with donkeys? Not a whole lot. Which is why sauter du coq à l’âne means to suddenly change the subject, jump from one subject to another, all the while adding a distinctive barnyard flavor to the conversation.
Sauter du coq à l'âne literally means to jump from the rooster to the donkey. Which begs the question: what do you do next? Jump to the horse, the sheep, the goat? Why did we start with the rooster in the first place? It turns out this truly odd idiom, which you will frequently hear in everyday francophone life, has a complicated history.
During the 14th century, people used the expression Saillir du coq à l’asne, to describe a rooster covering a duck - asne is an old word for duck. The scene in question here, if you can picture it, is that of rooster trying to jump on all the hens within his reach, and sometimes getting confused and choosing a duck as his favorite. This inconsistency passed in the human race as saillir du coq à l'asne to designate someone talking incoherently.
In time, people started to use this weird interracial fact as a comparison for people abruptly changing subject. Meanwhile, saillir, which meant cover or jump in old French, became sauter. Sauter du coq à l’asne was used to represent someone “jumping” from one subject to another completely unrelated one. By extension, you'll also hear passer du coq à l'âne, with the same meaning.
So, where is the donkey coming from?
To complicate matters, asne (you know, the old word for “duck“) and âne (donkey) are pronounced exactly the same. Asne is no longer used and has been replaced with la cane, the female of le canard. Except in today's idiom, where asne turned into âne instead. And that's how the donkey came into the picture. Simple as that.
On parlait de l’économie quand il est passé du coq à l’âne pour me demander où j'habitais
We were talking about the economy when he suddenly changed the subject and asked where I lived.
Il ne s'appuie aucunement sur les faits et passe constamment du coq à l'âne.
He lacks any basis in facts whatsoever and rambles from one area to the next.