Why does vulgarity relate to the mouth "ניבול פה" while evil gossip relates to the tongue "lashon hara"? הלא דבר הוא!!
When one says something negative about his or her friend the words themselves are not inherently evil "She is as bright as a doorknob", "He looks like a hyena". Each word is a kosher word. The problem is that it hurts the other person. It is like shooting an arrow at someone and harming them. The pasuk [Yirmiyahu 9/7] says "חץ שחוט לשונם" - their tongue slaughters like an arrow. That is what happens when one speaks ill of another. That explains why it is lashon hara.
When one uses foul language or talks about inappropriate matters then the very words that emanate from one's mouth are unclean even though they don't necessarily hurt anybody [like an arrow]. So it is nivul peh - a dead [like neveila - corpse, see Maharsha Pesachim 113] mouth. No life. No creativity. No goodness or G-dliness. No ויפח באפיו נשמת חיים -רוח ממללא. The gemara says that people die because of this sin. A dead mouth produces both death and turns around positive decrees to be evil ח"ו.
Let us see how the Maharal put it:
Weren't Chazal DEEEEEP??
