"You shall not curse the deaf"—Leviticus 19:14.
It is forbidden to curse any Jewish person.
Why then does the Torah specifically prohibit cursing the deaf?
When a person is angered, the measures the person will take to quiet his anger depends on the severity of the (perceived) offense. Sometimes, he will suffice with cursing and shaming the offender. For a greater offense, the person will not be satisfied until he destroys all the offender's property, or actually hurts or injures the individual. For the greatest of offenses, the victim's fury will not be placated until he kills the one who hurt him.
Then there's the smallest of angers. One that can be assuaged merely by cursing the offender—even while not in his presence, even if the offender will never become aware of the cursing.
By commanding us not to curse (even) the deaf, the Torah is telling us that it is forbidden to curse another even not in his presence, and even if he will never find out about the slight.
The reason for this mitzvah is that the Torah is concerned not only with the state of the one being cursed, but also with the state of the one who curses, and wants to ensure that he not become accustomed to harboring angry and vengeful thoughts.
Also included in this prohibition is that one may not even curse oneself.
"And he is witness, whether he has seen or known of it"—Leviticus 5:1.
We are commanded to testify before the judges regarding matters we are aware of, whether we actually saw an event or heard about it. This is true regardless whether the testimony will cause financial loss for the person regarding whom we testify, or whether the testimony can save a person or his possessions.
One who refrains from giving evidence, is guilty of a grave sin.
