Plato, in his Apology of Socrates, where he has Socrates use the “Unaccustomed as I Am to Public Speaking...“ tactic to preface his speech defending himself before the Athenian court. Socrates begins by saying that just as they would forgive a foreigner for speaking before them with an accent, he is so "foreign" to public speaking that they should forgive him if his rhetoric isn't very flowery. As you'd expect, an epic and eloquent rhetorical smackdown is about to ensue.
Frederick Douglass, before making a speech at an anti-slavery convention, apologized for being ignorant from being in slavery so long. He then proceeded to bring the house down with an eloquent, moving, and well-reasoned speech. In his case, he was just unsure of himself and being modest, and "ignorant" meant that he was unfamiliar with, say, Aristotle or Homer or Milton. Douglass was an avid reader who had self-directed much of his education.
There's a theory that Douglass was using Obfuscating Stupidity when he would say things like this (as he often did), to ensure that his white audience wouldn't just walk out or ignore everything he would say by insulting their "intelligence" by claiming to be an "intelligent black man" - something which simply did not exist as far as many whites of the time were concerned.