During the United States' war with Mexico, New Orleans was saturated with military activities.  The city had not seen that much war readiness involvement since the Battle of New Orleans.  In a strange way it seemed somewhat of a joyous time.  With spirits running high, military men became infatuated with a popular song entitled Green Gros.  They sang it every chance that presented itself.  They sang the song while they marched, and they sang it when they were not marching.  The Mexicans, hearing the song incessantly, began referring to all Americans as Green-gros, or in time, gringos.  After approximately 150 years, the term is still used when Mexicans refer to Americans.

Of course, this is all a nice urban fable as there is documented evidence that word existed well prior to the war.  The word is believed to be derived from the root for Greek, and means one who speaks Spanish with an accent.  There is still a Celtic connection as one theory holds that it was first used to describe the accent of the Irish who arrived in Spain during the Flight of the Earls.