How often we sin this way, without realizing the consequences! James in his Letter talks about how we sin with our tongues more than other parts of the body. Paul tells us if we can't say something in the name of the Lord, then we should not say it all. Slander is a form of gossip that is derogatory. It is negative and damages the reputation of the person spoken of. One confessor told a penitent who confessed this sin to take the feathers of a pillow and scattered them abroad and then come back to see him. When the person did, he told the penitent to go and collect the scattered feathers, which  was impossible. So it is with the slanderous words we utter. It is scattered far and wide. So to avoid this, follow the words of St. Anthony: if you can't say it in front of the person, don't say it in back of this person.