It’s a satisfying way to tell somebody that he’s stupid or ignorant, its Latinate form projecting an aura of dusty academic superiority. It also has a long and interesting history.
The ancient legal institution of the grand jury now continues only in the USA, but it was once the standard way of deciding whether a person should be charged with a crime. It was called a grand jury because it was made up of 24 men, twice the size of one in a trial, which was a petit jury or petty jury. Grand juries were originally called from among local men who were expected to act on personal knowledge. If they felt the evidence was too weak their foreman wrote the Latin word ignoramus on the back of the indictment. This literally meant “we do not know”, from the Latin verb ignorare, to be ignorant. In practice it meant “we take no notice of this”. It was the opposite of declaring the indictment a true bill, which meant the accused person went to trial.
How this abstruse foreign form from the specialised language of the law became an English word is due to George Ruggle. He wrote a play called Ignoramus, mostly in Latin, which was performed on 8 March 1615 at Trinity College, Cambridge, before an audience of some 2,000 which included King James I of England and the future Charles I. It featured a rascally and ignorant lawyer, the Ignoramus of the title, who used barbarous law Latin of a kind deplored by the university’s academics. The king loved the play but his judges and law officers hated it. It caused a huge controversy that led to the name of the play’s chief character entering the language.
Since there is no lack of ignorance and stupidity in our world, we have to decide how to create its plural. A slight knowledge of Latin noun plurals suggests it should be ignorami, to match nucleus, fungus, terminus, cactus, and stimulus. But ignoramus never was a Latin noun, so the sensible course is to stick to the rules of English, making ignoramuses. That’s a mouthful, but it will stop you from sounding like an ignoramus.