Late 14c., “sea monster, sea serpent,” sometimes regarded as a form of Satan, from Late Latin leviathan, from Hebrew livyathan “dragon, serpent, huge sea animal,” of unknown origin, perhaps from root l-w-h- “to wind, turn, twist,” on the notion of a serpent’s coils. If so, related to Hebrew liwyah “wreath,” Arabic lawa “to bend, twist.” Of powerful persons or things from c. 1600. Hobbes’s use is from 1651.
From Etymology Dictionary
An aquatic animal mentioned in the Old Testament. It is described in Job xli. apparently as a crocodile; in Isa. xxvii 1 it is called a piercing and a crooked serpent; and it is mentioned indefinitely in Ps. lxxiv. 14 as food and Ps. civ. 26. [Century Dictionary]
*LEVIATHAN is MORTAL