Erasmus, Adagia 1.37:
WITH A THICK MINERVA. WITH A FAT MINERVA. WITH A THICKER MUSE.
Minerva, according to the stories of poets, presides over arts and minds. From this came the phrase: Minerva unwilling. Beyond that, there was also the phrase with a fat or with a thick Minerva, which is indeed sometimes granted the solemn honor of being treated as a proverb. Columella, in the first chapter of his twelfth book of On Rural Matters, writes,
In this study of the country, however, scrupulosity of that sort is not examined, but as it is said, as long as he has a fat Minerva, a useful presage of a future storm will fall to the overseer.
Similarly, in the preface of the first book:
For agricultural matters can be administered neither by the subtlest nor on the other hand, as they say, by a fat Minerva.
And again, he also writes in the tenth book:
Nor is the subtlety of Hipparchus necessary to what they call the more fertile letters of rustic people.
That is said to occur with a fatter Minerva which occurs with less order, and with more simplicity, as if with less learning, and not with refined or exceptionally exacting care. Thus, when that Priapus, asks with naked words, though he could have sought it more urbanely through verbal convolutions, he says, ‘My Minerva is thick.’ And Horace, describing a philosopher instructed not in those precise reasonings and subtleties of the Stoics, but as if, without any art, expressing his philosophy according to his custom, and not so much learned as simple and sincere, says,
A rustic, irregularly wise and with a thick Minerva.
Aulus Gellius, in Attic Nights 14.1, writes,
Nevertheless, it was his opinion that in no way could that be comprehended and understood by however brilliant a human mind in such a brief and exiguous space of life, but that some few things were subject to mere conjecture and, if I may use his phrase, with a παχύτερον,
that is, more thickly and with a fat Minerva.
CRASSA MINERVA. PINGVI MINERVA. CRASSIORE MVSA
Minerua iuxta poetarum fabulas artibus atque ingeniis praesidet. Vnde et illud fluxit: Inuita Minerua. Praeterea illud Pingui seu crassa Minerua, quod quidem iam olim prouerbii vice celebratur. Columella libro De re
rustica duodecimo, capite primo.
In hac autem, inquit, ruris disciplina non consideratur eiusmodi scrupulositas, sed quod dicitur, pingui Minerua, quantumuis vtile continget villico tempestatis futurae praesagium.
Idem in primi libri praefatione:
Potest enim nec subtilissima nec rursum, quod aiunt, pingui Minerua res agrestis administrari.
Idem libro decimo:
Nec tamen Hipparci subtilitas pinguioribus, vt aiunt, rusticorum literis necessaria est.
Dicitur pinguiore Minerua fieri, quod inconditius simpliciusque quasique indoctius fit, non autem exquisita arte nec exactissima cura. Vnde et Priapus ille, cum rem obscoenam, quam poterat vrbanius per inuolucra verborum petere, nudis verbis rogat, Crassa, inquit, Minerua mea est. Et Horatius philosophum describens non exactis illis Stoicorum rationibus atque argutiis instructum, sed veluti citra artem philosophiam moribus exprimentem neque tam disertum quam simplicem ac syncerum,
Rusticus, inquit, anormis sapiens crassaque Minerua.
Aulus Gellius lib. xiiii., cap. i.:
Nequaquam tamen id censebat in tam breui exiguoque vitae spatio, quantouis hominis ingenio comprehendi posse et percipi, sed coniectari pauca quaedam et, vt verbo ipsius vtar, παχύτερον, id est crassius et pingui Minerua.
 Erasmus is stretching the application of this excerpt.