The Collaborative International Dictionary of English v.0.48:

Fanega \Fa*ne"ga\, n. [Sp.]
   A dry measure in Spain and Spanish America, varying from 1?
   to 2? bushels; also, a measure of land. --De Colange.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

FANEGA, Spanish law. A measure of land, which is not the same in every
province. Diccionario de la Acad.; 2 White's Coll. 49. In Spanish America,
the fanega consisted of six thousand and four hundred square varas or yards.
2 White's Coll. 138.