1570s, originally an Anglo-French legal term (early 15c.), from Latin ignoramus "we take no notice of, we do not know," first person plural present indicative of ignorare "not to know, take no notice of" (see ignorant). The legal term was one a grand jury could write on a bill when it considered the prosecution's evidence insufficient. Sense of "ignorant person" (1616) came from the title role in George Ruggle's 1615 play in Latin satirizing the ignorance of common lawyers. The plural is ignoramuses as it never was a noun in Latin.