Laudator Temporis Acti

Quintilian 1.5.57 (tr. H.E. Butler):

I have heard that gurdus, which is colloquially used in the sense of "stupid," is derived from Spain.

gurdos, quos pro stolidis accipit vulgus, ex Hispania duxisse originem audivi.

Alessandro Garcea and Valeria Lomanto, "Gellius and Fronto on Loanwords and Literary Models: Their Evaluation of Laberius," in Leofranc Holford-Strevens and Amiel Vardi, edd., The Worlds of Aulus Gellius (Oxford: Oxford University Press, 2004), pp. 41-64 (at 59, n. 55):

On Quintilian’s testimony, which has long been debated by both literary and linguistic scholars, see e.g. F. Schöll, 'Wortforschung', 313–17, and J. Cousin, 'Problèmes', 63–4: both scholars acknowledge the vulgar connotation of gurdus, but while the former allows its Spanish origin, the latter doubts it.

The references are to Fritz Schöll, "Zur lateinischen Wortforschung," Indogermanische Forschungen 31 (1912-1913) 309-320, and Jean Cousin, "Problèmes biographiques et littéraires relatifs à Quintilien," Revue des Etudes Latines 9 (1931) 62–76.

Thesaurus Linguae Latinae 6,2:2359: A. Walde and J.B. Hoffmann, Lateinisches etymologisches Wörterbuch, 3. Aufl., Bd. I (Heidelberg: Carl Winter, 1938), p. 627: Alfred Ernout and Alfred Meillet, Dictionnaire étymologique de la langue latine, 4th ed. rev. Jacques André (Paris: Klincksieck, 2001), p. 285: Michiel de Vaan, Etymological Dictionary of Latin and the Other Italic Languages (Leiden: Brill, 2008), p. 275: See also Nicholas Zair, "Latin bardus and gurdus," Glotta 94 (2018) 311-318 (at 314-317).