Up until the early 1600s, the word ignoramus literally referred to the plural form of the Latin ignoro, meaning “I don’t know”. Ignoramus (“we don’t know”) was written by grand juries on the backs of indictments if there was insufficient evidence to proceed with a case.
The present day popular meaning of the word (any ignorant person or a dunce) was made popular in 1615 by George Ruggle in his satirical play, named after its main character, Ignoramus – a lawyer who doesn’t know anything about the law. The play is considered by many to be the most famous and influential academic play of English Renaissance drama. (Wikipedia, QPB Encyclopedia of Word and Phrase Origins)