World Wide Study Bible
a Bible passage
9. And he asked him, What is thy name?—The object of this question was to extort an acknowledgment of the virulence of demoniacal power by which this victim was enthralled.
And he answered, saying, My name is Legion: for we are many—or, as in Luke (Lu 8:30) "because many devils [demons] were entered into him." A legion, in the Roman army, amounted, at its full complement, to six thousand; but here the word is used, as such words with us, and even this one, for an indefinitely large number—large enough however to rush, as soon as permission was given, into two thousand swine and destroy them.