a fair-skinned foreigner whose native language is not Spanish; an American (a person from the United States), especially a white American.
“... gringos, llaman en Málaga a los extranjeros, que tienen cierta especie de acento, que los priva de una locución fácil, y natural Castellana; y en Madrid dan el mismo, y por la misma causa con particularidad a los irlandeses.” — gringos is what, in Malaga, they call foreigners who have a certain kind of accent that prevents them from speaking Castilian easily and naturally; and in Madrid they give the same name, in particular, to the Irish.
Synonyms: gabacho, guiri
Possibly from griego (“Greek”), particularly from the phrase hablar en griego (“to speak Greek”), with a similar connotation to the English phrase it's all Greek to me. Possibly influenced by peregrino (“pilgrim”).