IN Rabelais's Tiers livre, published in 1546, Frere Jan counsels Panurge regarding the desirability of his marrying immediately and then copulating with great frequency:
Seulement ayez esguard et consyderation de tous jours bien lier et continuer tes coups. Si tu y fays intermission, tu es perdu, paouvret, et t'adviendra ce que advient es nourrisses. Si elles desistent alaicter enfans, elles perdent leur laict. Si continuellement ne exercez ta mentule, elle perdra son laict et ne te servira que de pissotiere....  Je t'en advise, mon amy. J'en ay veu l'experience en plusieurs qui ne l'ont peu quand ilz vouloient, car ne l'avoient faict quand le povoient. Aussi par non usaige sont perduz tous privileges, ce disent les clercs. (Rabelais 5:208)
The best English translators have been uncertain whether the French word clercs should be made to refer to scholars or (more specifically) to clergymen. Donald Frame translates, "Thus by disuse are lost all privileges, so say the clerics" (Frame 337). Burton Raffel makes the other choice: "And as the scholars say, if you don't use it, you lose it" (Raffel 313). J. M. Cohen skirts the choice by translating very literally (and not very idiomatically in modern English): "Thus by lack of usage, as the clerks say, all privileges are lost" (Cohen 363). M. A. Screech is more definite: "As the law-clerks say: All privileges are lost by non-usage," adding the notation "'Privileges are lost by non-usage' was a legal maxim.'" (Screech, Gargantua 512-13; italics in the text). Jacques LeClercq, by implication, also understands his namesake clercs to refer to lawyers--even more specifically, canon lawyers: "An ancient maxim of cannon [sic!] law tells us that nonusage of full privileges makes them forfeit" (LeClercq 392). Of course, in the early sixteenth century most scholars were, in fact, clergymen of some description, and for many of them the study of both canon law and civil law would have belonged to the preparatory curriculum--as occurred in the case of Rabelais himself.
Just what are the status and role of the quoted pronouncement by les clercs, which Jacques LeClercq calls "an ancient maxim" and Burton Raffel has rendered with a version of the modern English proverb "Use it or lose it"? Let us look at the passage as given in the classic translation by the Scotsman Sir Thomas Urquhart, which appeared posthumously in 1693: Friar John says,
This is a certain truth I tell thee, Friend, and doubt not of it; for my self have seen the sad experiment thereof in many, who cannot now do what they would, because before they did not what they might have done. Ex desuetudine amittuntur Privilegia. Non-usage oftentimes destroys ones Right, say the learned Doctors of the Law. (Urquhart 2:131; italics in the text).
Especially in the Latin form (literally, "From disuse are lost privileges"), the opinion that Urquhart's Friar John attributes to the "Doctors of the Law" is presented as if it will be recognized as a common aphorism. Numerous twentieth-century books of sayings or quotations (including several published in countries where neither English nor French--nor Latin--is widely spoken) designate or categorize the Latin sentence as a legal maxim or simply a maxim--sometimes, even, a proverb.
Obviously, "Ex desuetudine amittuntur privilegia" does not appear in Rabelais's original text of 1546. In fact, I cannot discover any occurrence of the Latin sentence prior to Urquhart's interpolation of it in 1693 - or any more recent occurrence, outside the published collections of sayings and a few direct (and usually inaccurate) references to Rabelais. Rabelais had written only "Aussi par non usaige sont perduz tous privileges, ce disent les clercs." A footnote in the standard edition of Rabelais (by Lefranc et al., 1931) calls the French saying an "ancienne maxime de droit canonique" (Rabelais 5:208). In his modern scholarly edition of Le tiers livre, M. …