The Collaborative International Dictionary of English v.0.48:

Ignoramus \Ig`no*ra"mus\, n. [L., we are ignorant. See
   Ignore.]
   1. (Law) We are ignorant; we ignore; -- being the word
      formerly written on a bill of indictment by a grand jury
      when there was not sufficient evidence to warrant them in
      finding it a true bill. The phrase now used is, "No bill,"
      "No true bill," or "Not found," though in some
      jurisdictions "Ignored" is still used. --Wharton (Law
      Dict. ). Burn.
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   2. (pl. Ignoramuses.) A stupid, ignorant person; a vain
      pretender to knowledge; a dunce.
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            An ignoramus in place and power.      --South.
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