I've heard experts voice that comes from protestis, term used prior to Christianity when to take up a post man played with one hand and with the other testicle lifted his arm as it is now customary in that Protocol to say: Yes, protestis. That meant that if you do not comply with the encomienda fully put at risk his testicles.Later, when women began to exercise public office changed the term: Yes, protest, which comes from protest, that as meaning we interpretaríamos "By my head ".I suggest more research in this regard, that listened.