By “cussing” you mean angry or blithe use of foul language?
I think that “cussing” is sinful, but less than what pious people frequently think. One of my clients in an auto accident case was a “cussing” old lady with a really big, wonderful, generous, happy heart. One of her acquaintances was a very severe, Bible-pounding non-Catholic Christian who acted scandalized whenever the “cussing” took place. One day, the “cussing” lady was giving the Bible-pounder a ride to the doctor. They came to an intersection controlled by a traffic light. The “cussing” lady had the green. A car on the cross street went through the red light at a high speed. The “cussing” lady’s car caught the other car on the rear, just enough to turn it leftward so that it slammed beneath a tractor-trailer trailer. The driver-husband was decapitated. The passenger wife ducked, and filed criminal charges against innocent “cussing” lady.
When I was retained to represent the “cussing” lady, I paid a visit to the Bible-pounder to see if she would be a witness on her behalf. The Bible-pounder said, “I guarantee you that she is innocent. We definitely had the green. But I’m not going to testify. It’s against the Bible.”
I said, "The Bible says that you should not bear false witness against thy neighbor. It doesn’t say, “Don’t bear witness.”
The Bible-pounding lady immediately became enraged and screamed at me to leave her house and stop violating her religious rights.
I’ll take the big-hearted “cussing” lady over the nasty, selfish Bible-pounder any day.
Things have to be looked-at in context.
I think that, generally, “cussing” amounts to a small comitment of the heart to “the lower things.” The eyes are the window of the soul, but where the mouth goes, the brain also tends to follow. So, it is a little bit sinful. Most are unaware, however, that God himself uses foul language in the Bible as He fulminates nastily against His people’s sins. E.g., Lamentations 1:9.**