The Collaborative International Dictionary of English v.0.48:

Hidalgo \Hi*dal"go\, n. [Sp., contr. fr. hijo de algo, i. e.,
   son of something; hijo son (fr. LL. filius) + algo something,
   fr. L. aliquod. Cf. Fidalgo.]
   A title, denoting a Spanish nobleman of the lower class.
   [1913 Webster]