jackass

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

• noun A male ass or donkey.
• noun A foolish or stupid person; a blockhead.

from The Century Dictionary.

• noun A male ass; a jack.
• noun Hence A very stupid or ignorant person: used in contempt.
• noun Nautical, same as hawse-bag.

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

• noun The male ass; a donkey.
• noun A conceited dolt; a perverse blockhead; -- disparaging and offensive.
• noun (Naut.) a three-masted vessel, with only the foremast square-rigged; a barkentine.
• noun (Zoöl.) the koba.
• noun (Zoöl.) See Jack rabbit, under 2d Jack, n.
• noun (Zoöl.) any species of penguin of the genus Spheniscus, of which several are known. One species (Spheniscus demersus) inhabits the islands near the Cape of Good Hope; another (Spheniscus Magellanicus) is found at the Falkland Islands. They make a noise like the braying of an ass; -- hence the name.
• noun (Zoöl.) See under Laughing.

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

• noun A male donkey.
• noun A foolish or stupid person.
• noun An inappropriately rude or obnoxious person.
• proper noun poker slang A jack and an ace as a starting hand in Texas hold 'em due to phonetic similarity
• verb rare To behave very obnoxiously.

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

• noun male donkey
• noun a man who is a stupid incompetent fool