I've never heard it said that "Serb" meant "sir."
The late Albert Bates Lord, better known for his work on oral poetry and Homer, recorded Serb, Bosnian, and Albanian oral poets in the former Yugoslavia (initially as an assistant to Milman Parry but he continued long after Parry's death in 1935). He wrote an introduction to the Serbo-Croatian language which has reading selections from a novel (in Serbian) which compared a Serb in Serbia to a Boer (this was at the time of the Boer War--one point of comparison was Serbia's struggle against the Turks being analogous to the Boers' struggle against the English, but this particular man was being compared to a Boer because he had a very large family like the Boers of that day). I forget the title--maybe something like "Boers and Englishmen."
I was not saying that you said that Serb meant sir but was just using an analogy because reducing a term to its roots often diminishes the fact that it might have since been expanded to denote an entire people or nation.