But the things that come out of a person’s mouth come from the heart, and these defile them. For out of the heart come evil thoughts—murder, adultery, sexual immorality, theft, false testimony, slander. These are what defile a person... Matthew 15:18-20
Words are not "bad" per se. Words have evolved over centuries. Some words spoken in public today, by pastors, would have been considered scandalous fifty years ago. The world has had a part in it, being more accepting of cursing, but it's really the intent of the one saying the word that is the issue.
Jesus made it clear that it was not just what came out of a person's mouth, but what came out of his heart that defiled him. Bad words are bad because they express the hate or sexuality of the heart they came from.
Finally, brothers and sisters, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things. Whatever you have learned or received or heard from me, or seen in me—put it into practice. And the God of peace will be with you. Philippians 4:8-9
God asks that our hearts reflect on the good and that our mouths express the godly. We should not say something just because its acceptable by the world or "cool," but because it reflects a heart like God's.
Certain words should not be used because they stir negative responses and ungodly thoughts in other people. They should not be used because they cause others to sin or stir hate in their hearts.
Bad words are bad because they reflect an ungodly heart.
Bad words are bad because they stir ungodly thoughts in others.
