Brown Driver Briggs

 1. leviathan, sea monster, dragon
  1) large aquatic animal
  2) perhaps the extinct dinosaur, plesiosaurus, exact meaning unknown

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Some think this to be a crocodile but from the description in
Job 41 this is patently absurd. It appears to ba a large fire breathing
animal of some sort. Just as the bomardier beetle has an explosion
producing mechanism, so the great sea dragon may have an explosive
producing mechanism to enable it to be a real fire breathing dragon.