An ignoramus is someone who lacks knowledge or understanding, especially when they should know better. The word carries a sharp edge of criticism: it doesn't just mean someone doesn't know something; it suggests someone who is proudly or stubbornly uninformed.

If a student insists the Earth is flat despite learning about astronomy in school, a frustrated teacher might think of them as an ignoramus. If someone confidently declares that George Washington invented the telephone, they're revealing themselves to be an ignoramus about American history.

Notice that calling someone an ignoramus is much harsher than saying they're simply ignorant (lacking knowledge) or mistaken (wrong about something). It suggests willful arrogance about one's own lack of knowledge. Use this word carefully: it's an insult that stings because it questions both someone's intelligence and their attitude about learning.