From lavah; a wreathed animal, i.e. A serpent (especially the crocodile or some other large sea- monster); figuratively, the constellation of the dragon; also as a symbol of Bab. -- leviathan, mourning.
KJV: In that day the LORD with his sore and great and strong sword shall punish leviathan the piercing serpent, even leviathan that crooked serpent; and he shall slay the dragon that is in the sea.
NASB: In that day the LORD will punish Leviathan the fleeing serpent, With His fierce and great and mighty sword, Even Leviathan the twisted serpent; And He will kill the dragon who lives in the sea.