Erasmus, Adagia 1.39:

Less Cultivated and More Clearly:

Indeed, that phrase is put less elegantly by the Greeks, but it has the same force: Ἀμαθέστερον καὶ σαφέστερον εἰπέ [speak less learnedly and more clearly], which is found in Gellius as well. He says,

‘For you know, I think, that ancient and commonly circulated phrase, Ἀμαθέστερον καὶ σαφέστερον εἰπέ,’

that is, Speak less learnedly and more plainly, and say it more openly and clearly. It appears to be taken from from a comedy of Aristophanes, titled Βάτραχοι, that is, The Frogs:

Ἀμαθέστερον πως εἰπὲ καὶ σαφέστερον,

that is, Speak less learnedly and more clearly. In this song, Bacchus chides the obscurity of Euripides, who had proposed something or other with insufficient lucidity. Suidas and an interpreter advise us that there is a proverb underlying it, which runs:

Σαφέστερόν μοι κἀμαθέστερον φράσον,

that is, Speak to me more openly and less learnedly. I suspect that it was taken from the fact that in antiquity, those sophists (as they call them) were accustomed to exert a fair amount of labor in covering over the mysteries of wisdom with certain enigmatical entanglements, clearly with the intention of keeping the profane mob not yet initiated into the sacred secrets of philosophy from following it. Nay, even today, some professors of philosophy and theology, when they are about to relate what any little old lady or workman might say, tangle and wrap up the matter with little spikes and portents of words so that they will seem learned. Thus Plato with his numbers obscured his own philosophy. Thus Aristotle, with all of his learned collections, made a lot of things more obscure.

RVDIVS AC PLANIVS     

Inelegantius quidem est illud apud Graecos, sed idem tamen pollet: Ἀμαθέστερον καὶ σαφέστερον εἰπέ, quod apud eundem refertur Gellium.

Nosti enim, inquit, credo, verbum illud vetus et peruulgatum, μαθέστερον εἰπὲ καὶ σαφέστερον,

id est Indoctius rudiusque quodammodo loquere et apertius ac clarius fare. Sumptum apparet ex Aristophanis comoedia, cui titulus Βάτραχοι, id est Ranae:

Ἀμαθέστερον πως εἰπὲ καὶ σαφέστερον,

id est  Indoctius proloquitor atque clarius. Quo carmine Bacchus Euripidis obscuritatem taxat, qui nescio quid parum dilucide proposuerat. Suidas et interpres admonent subesse prouerbium, quod hunc ad modum feratur:

Σαφέστερόν μοι κἀμαθέστερον φράσον,

id est  Apertius mihi loquere atque indoctius. Suspicor inde sumptum, quod antiquitus illi σοφοί, quos vocant, soleant mysteria sapientiae quibusdam aenigmatum inuolucris data opera obtegere, videlicet ne prophana turba ac nondum philosophiae sacris initiata posset assequi. Quin et hodie nonnulli philosophiae ac theologiae professores, cum ea quandoque tradant, quae quaeuis muliercula aut cerdo dicturus sit, tamen quo docti videantur, rem spinis quibusdam ac verborum portentis implicant et inuoluunt. Sic Plato numeris suis obscurauit suam philosophiam. Sic Aristoteles multa mathematicis collationibus reddidit obscuriora.