I believe it is the opposite of derogatory, actually, but includes a derogatory implication upon those who are not "hidalgo". Confused? LOL! Well, the "son of something" is as opposed to a "son of nothing", the "nothing" being the common people. Hence, it refers to nobility. It was probably a further shortening of a phrase, such as "hijo de algo importante". The word for someone is close, "alguien", and perhaps the original phrase used it, but phonemically it is very different. Phonemically "nothing" (nada) and "no one" (nadie) are much closer, but curiously, there is no word deriving from "son of nothing". Does that help? :0)