Since the twelfth century, the phrase fijo d'algo (lit. son of something, assuming that algo is not also a contraction) and its contraction, fidalgo, were used in the Kingdom of Castile and in the Kingdom of Portugal to identify a type of nobility. In Portugal, the cognate remained fidalgo, which identified nobles of a similar status to a hidalgo in Spain. In the Kingdom of Aragón, the infanzón was the noble counterpart of the Castilian hidalgo. The pronunciation changes in Spanish occurred during the late Middle Ages, the letter-F sounding was lost, and replaced with the letter-H spelling and pronunciation of hidalgo. (see History of the Spanish language)
The origin of the word "hidalgo" is disputed. The word may be a contraction of hijo de la Godo, or son of the Goth. When the Goths invaded the Iberian Peninsula, they supplanted the previous social structure and became the ruling class. Thus to be the son of a Goth would indicate one was upper class gentility.
In time, the term included the lower-ranking gentry, the untitled, lower stratum of the nobility who were exempted from taxation. The Siete Partidas (Leyes de Partidas), suggests that the word hidalgo derives from itálico ("italic"), a man with full Roman citizenship.
In the previous Visigoth monarchies, the condition of the hidalgo was that of a freeman without land wealth, but with the nobleman's rights to wear arms and to be exempt from taxation, in compensation for military service; the military obligation and the social condition remained in force by the Fuero Juzgo law.