Hilchos Lashon Hara, Opening – negative commands (10)

Sometimes a person stands up in beis din (a Jewish court) and testifies alone about a sin someone else did. Since he is only one witness, his testimony cannot do anything useful: it cannot make the person pay money or take an oath, and it cannot take away the person's status as a trustworthy (kosher) Jew. So all he does is give the person a bad name.

He breaks the negative commandment (Devarim 19:15), "A single witness shall not stand [to give witness] about a man regarding any sin or any transgression." The beis din must give him lashes for this.