An insulting word for someone who does not know things that most people around them know — and, usually, who is confident anyway. That second part is why people choose this word instead of just 'he doesn't know'. It carries a sneer, and it makes the speaker sound a bit grand, so it often lands as old-fashioned or mock-formal. You will see it in angry newspaper columns, in comment sections, and in comedy. Because it sounds pompous, English speakers frequently use it about themselves, as a joke and a way of being modest: 'I'm a complete ignoramus when it comes to cars.' That self-mocking use, often with 'a complete / total ignoramus about ___', may be the most common one in real speech, and it is quite friendly. Aimed at another person, though, it is a real insult. The plural is 'ignoramuses'. Do not write 'ignorami' — it looks Latin, but the original word was not that kind of noun, so that plural is a joke form only.