The Collaborative International Dictionary of English v.0.48:

Gringo \Grin"go\, n. [Amer. Sp., fr. Sp. gringo gibberish; cf.
   griego Greek, F. grigou wretch.]
   Among Spanish Americans, a foreigner, especially an American
   or sometimes an Englishman; -- often used disparagingly or as
   a term of reproach.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

gringo
    n 1: a Latin American (disparaging) term for foreigners
         (especially Americans and Englishmen)