What the Bible says about Filthy Language
(From Forerunner Commentary)
The demon is called an "unclean" spirit, "impure" and "lewd." In Scripture, demons are well-known for being unclean, as representing the work of sin, which defiles the sinner. Curse words are "filthy language," and some people love to tell "dirty jokes." Moral filth often goes along with physical filth; evil people are dirty in mind and often in body and dwelling (Isaiah 28:6-8).
The Bible describes demons abusing those they possessed by causing them to convulse, fall, break a limb, foam at the mouth, roll, and gnash their teeth. The words "suffers severely" (Matthew 17:15) come from two Greek words that mean "badly" and "experience." To be controlled by sin is a horrible experience; it is a brutal, distressing taskmaster. Sin's delights are brief and small (Hebrews 11:24-26), while its severe discomforts can last a lifetime.
Martin G. Collins
The Miracles of Jesus Christ: Exorcising a Young Boy (Part Two)