While reading about Diderot’s correspondence, as part of the process of digitizing an edition, I saw the claim in several places that the word ‘calembour’ was coined by Diderot, and first used in a letter to Sophie Volland on 1st October 1768. Out of interest, I searched for the term in Electronic Enlightenment – a major collection of letters from this period – and found an instance of Madame de Graffigny using it in a letter of 1746 (https://doi.org/10.13051/ee:doc/graffrVF0081114a1c). The meaning in this case appears to be the same as the Diderot’s, i.e. a pun, or joke based on the potential double meaning of the sound of a word. Other examples were later than 1768.
I looked at some historical dictionaries, and the Trésor de la Langue Francais cites Diderot as the first source. You can see all this at https://www.cnrtl.fr/definition/dmf/calembour. Here, I also saw the alternative spellings ‘calambour’ and ‘calembourg’, so I searched Electronic Enlightenment for these (and searched for plurals as well), and discovered seven more uses of the term pre-dating 1768, all by Graffigny.
I note also that the Base Historique du Vocabulaire Français has a citation from a work by Vadé published in 1757, but maybe written earlier. (See https://www.cnrtl.fr/definition/bhvf/calembour).
The earliest Graffigny citation is below. I note that she doesn’t have to explain what a ‘calambour’ is. I don’t quite understand the joke, but it does appear to be one, and one dependent on double meanings.
Je veux remplir cette page et je n’ai plus qu’un calambour, que je trouve exellent. C’est un Mr qui au beau milieu d’une compagnie se leve gravement et va chercher tout autour des pieds d’une dame comme s’il avoit perdu quelque chose. La dame lui demande ce qu’il cherche. «Des trufes, madame,» dit-il. Or devine le calambour : c’est une galanterie. Si tu jette ta langue au chien, je te le dirai. C’est Du Mai qui me l’a conté et qui avoit eté temoin de ce joli jeu. [https://doi.org/10.13051/ee:doc/graffrVF0020255a1c]
So it looks like, thanks to the digitized letters in Electronic Enlightenment, I might have been able to find an use of a neologism in French earlier than those attested in dictionary sources. This may be of interest to a Graffigny scholar, as well as to historical linguists and lexicographers.
As a postscript, I will add that I also recently came across the toponym ‘Calenburg’ in Richard Ovenden’s recent work ‘Burning the Books‘ (p.55). It appears to be a place of some historical significance in Lower Saxony, and it’s not impossible that it is the source of the term, but I haven’t found any evidence yet for a connection.