Thayer's Greek Lexicon

 1. prone to slander, slanderous, accusing falsely
  1) a calumniator, false accuser, slanderer,
2. metaph. applied to a man who, by opposing the cause of God, may be
said to act the part of the devil or to side with him

Satan the prince of the demons, the author of evil, persecuting good
men, estranging mankind from God and enticing them to sin,
afflicting them with diseases by means of demons who take possession
of their bodies at his bidding.