Thomas over at Jihadica has a lot more on the term ruwaybidha, including one of the traditions from which some version of the term stems, also politely correcting my original transliteration. I still think that the term implies a sense of authority or responsibility, almost like a shepherd (al-rabidha), coming from rabidh (herd of sheep or goats). Al-Ruwaybidha, then would be the diminutive of al-rabidha and, of course, an insult.