Yours, Etc.: Origins and Uses of 8 Sign-Offs
1570s, from an Anglo-French legal term (early 15c.), from Latin ignoramus "we do not know," first person present indicative of ignorare "not to know" (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" came from the title role of George Ruggle's 1615 play satirizing the ignorance of common lawyers.