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]

Moby Thesaurus II by Grady Ward, 1.0:

67 Moby Thesaurus words for "ignoramus":
   abecedarian, alphabetarian, apprentice, articled clerk, ass,
   beginner, boot, born fool, buffoon, catechumen, clown, dabbler,
   debutant, dilettante, doodle, dullard, dullhead, dumbbell,
   dummkopf, dummy, dunce, egregious ass, entrant, figure of fun,
   fledgling, fool, freshman, greenhorn, greeny, idiot, illiterate,
   illiterati, inductee, initiate, jackass, know-nothing, lowbrow,
   lunatic, middlebrow, milksop, mooncalf, moron, neophyte, new boy,
   newcomer, no scholar, novice, novitiate, perfect fool, postulant,
   probationer, probationist, puddinghead, raw recruit, recruit,
   rookie, schmuck, simpleton, softhead, sop, stupid, stupid ass,
   tenderfoot, tomfool, tyro, unintelligentsia, zany



Bouvier's Law Dictionary, Revised 6th Ed (1856):

IGNORAMUS, practice. We are ignorant. This word, which in law means we are
uninformed, is written on a bill by a grand jury, when they find that there
is not sufficient evidence to authorize their finding it a true bill.
Sometimes, instead of using this word, the grand jury endorse on the bill,
"Not found." 4 Bl. Com. 305. Vide Grand Jury.



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