I have always used the word "Ignorant" to mean "not having knowledge of" as opposed to being an insult. I am ignorant when it comes to speaking Spanish. I am ignorant with regard to "String Theory" and "Quantum Physics". While I am ignorant with regard to these things, that doesn't mean I am incapable of learning them. If I want to insult someone or I think they are incapable of learning, then that is different than "Ignorant". For that I reserve the word "Stupid". Just my $0.02