The only free and comprehensive online etymological dictionary of the Spanish language
gordo (Adjective) "fat"
12th cent. From Latin gurdus "heavy," "stupid." Even as late as the 18th cent., gordo could mean dull-witted. Of unknown origin. Borrowed from a non-Indo-European language in Iberia.
Also the origin of the surnames Gordo, Gordillo, Gordilla, Gordito, Gorejo, Gordjuela, Gordón, and Gordoncillo; originally used as names for overweight individuals and later fossilized into surnames.
Indo-European
Romance
Asturian gordu, Portuguese gordo, Catalan gord, French gourd

Gorda or perra gorda "10 cent coin," c. 1870, an uncommon name first made in reference to the 10 cent coins minted by the the provisional government of Spain of 1868-1871. The coin featured a lion with an unclear design that resembled more a fat dog than a stately lion. As the design has long since been retired, the name perra gorda has outlived the coin.

"Furthermore - returning to my original plan - words are either Latin or foreign. Foreign words, just like people, have come to us from almost every nation. ...I have heard that gurdus, the vulgar word for "fool," comes from Spain." ~ Quintilian, De institutione oratorio (in The Orator's Education 2002)