In 1615 in England, a play called "Ignoramus" was first producedd. This play, a farce written by George Ruggle, is set at Cambridge University, and has as the title character a magistrate who believes himself to be shrewd when in fact he is foolish and ignorant.
In Latin, "ignoramus" means "we do not know." In Ruggle's time the Latin word was used in legal proceedings in England. Grand juries would write "ignoramus" on bills of indictment when the presented evidence was insufficient to justify prosecution. It seems likely that it was from this legal use that Ruggle took his character's name, especially when we consider that the "Ignoramus" of Ruggle's play was based on an actual Cambridge magistrate who was involved in a legal feud between the town of Cambridge and the university.Ruggle's play was immediately successful, and it was not long before "ignoramus" came to be used as a contemptuaous term for any wretchedly ignorant person, say editors at Merriam Webster Inc.