The Collaborative International Dictionary of English v.0.48:

Jackass \Jack"ass`\, n. [2d jack + ass.]
   [1913 Webster]
   1. The male ass; a donkey.
      [1913 Webster]

   2. A conceited dolt; a perverse blockhead; -- disparaging and
      offensive.
      [1913 Webster]

   Jackass bark (Naut.), a three-masted vessel, with only the
      foremast square-rigged; a barkentine.

   Jackass deer (Zool.), the koba.

   Jackass hare, Jackass rabbit (Zool.). See Jack rabbit,
      under 2d Jack, n.

   Jackass penguin (Zool.), 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.

   Laughing jackass. (Zool.) See under Laughing.
      [1913 Webster]

WordNet (r) 3.0 (2006):

jackass
    n 1: a man who is a stupid incompetent fool [syn: fathead,
         goof, goofball, bozo, jackass, goose, cuckoo,
         twat, zany]
    2: male donkey [syn: jack, jackass]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "jackass":
   Rocky Mountain canary, ass, born fool, buffoon, burro, clown,
   cuddy, dickey, donkey, doodle, egregious ass, figure of fun, fool,
   idiot, ignoramus, imbecile, jack, jennet, jenny, jenny ass, jerk,
   lunatic, milksop, mooncalf, neddy, nincompoop, perfect fool,
   schmuck, stupid ass, tomfool, zany