ignoramus

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An ignorant person.

Century Dictionary and Cyclopedia

1. n. 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. It is now superseded in some States by the phrase “not a true bill,” or “not found”; but the jury is still said to ignore the bill or the indictment. The indorsement “ignoramus” on a bill returned by a grand jury properly implied no more than that the jury deemed it inexpedient to pursue the matter; but it was often taken as an indication of ignorance or stupidity on the part of the jury, thus leading to the present familiar use as an English noun. Also used attributively.
2. n. n. An ignorant person; especially, one who lacks necessary knowledge; an ignorant pretender to knowledge.

Wiktionary

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

GNU Webster's 1913

1. n. 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.
2. n. A stupid, ignorant person; a vain pretender to knowledge; a dunce.

WordNet 3.0

1. n. an ignorant person