Refers to someone who comes from the French Basque country and were seen as "other" by the Spaniards when they arrived in Spain
in the middle ages. So, as the language evolved, and the Spaniards wished to speak disparagingly of a foreigner, they might call him a Gavacho. When the indigenous people of Mexico were learning Spanish, and heard the word, they
incorporated it into the language too. Though the true
etymology of the word refers to a person of French heritage from the Basque region it is now commonly used to refer to any and all Anglo-Americans.