ignoramus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An ignorant person.

from The Century Dictionary.

• noun In law, an indorsement, meaning ‘we ignore it,’ which a grand jury formerly made on a bill presented to it for inquiry, when there was not evidence to support the charges, by virtue of which indorsement all proceedings were stopped, and the accused person was discharged.
• noun n. An ignorant person; especially, one who lacks necessary knowledge; an ignorant pretender to knowledge.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.
• noun A stupid, ignorant person; a vain pretender to knowledge; a dunce.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A totally ignorant person—unknowledgeable, uneducated, or uninformed; a fool.
• noun law, dated A grand jury's ruling on an indictment when the evidence is determined to be insufficient to send the case to trial.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an ignorant person