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.
      [1913 Webster]

   2. (pl. Ignoramuses.) A stupid, ignorant person; a vain
      pretender to knowledge; a dunce.
      [1913 Webster]

            An ignoramus in place and power.      --South.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ignoramus
    n 1: an ignorant person [syn: ignoramus, know nothing,
         uneducated person]

The Devil's Dictionary (1881-1906):

IGNORAMUS, n.  A person unacquainted with certain kinds of knowledge
familiar to yourself, and having certain other kinds that you know
nothing about.

    Dumble was an ignoramus,
    Mumble was for learning famous.
    Mumble said one day to Dumble:
    "Ignorance should be more humble.
    Not a spark have you of knowledge
    That was got in any college."
    Dumble said to Mumble:  "Truly
    You're self-satisfied unduly.
    Of things in college I'm denied
    A knowledge -- you of all beside."
                                                               Borelli