Q: What is forbidden in the ninth commandment?
A: The ninth commandment forbiddeth whatsoever is prejudicial to truth, or injurious to our own, or our neighbour's good name.

Proverbs 19:5. A false witness shall not be unpunished, and he that speaketh lies shall not escape.
Proverbs 6:16-19. These six things doth the Lord hate: yea, seven are an abomination unto him: a proud look, a lying tongue, and hands that shed innocent blood, an heart that deviseth wicked imaginations, feet that be swift in running to mischief, a false witness thatspeaketh lies, and he that soweth discord among brethren.
1 Samuel 17:28. And Eliab his eldest brother heard when he spake unto the men; and Eliab's anger was kindled against David, and he said, Why camest thou down hither? and with whom hast thou left those few sheep in the wilderness? I know thy pride, and the naughtiness of thine heart; for thou art come down that thou mightest see the battle.
Leviticus 19:16. Thou shalt not go up and down as a talebearer among thy people: neither shalt thou stand against the blood of thy neighbour: I am the Lord.
Luke 3:14. And the soldiers likewise demanded of him, saying, And what shall we do? And he said unto them, Do violence to no man, neither accuse any falsely; and be content with your wages.
Psalm 15:3. He that backbiteth not with his tongue, nor doeth evil to his neighbour, nor taketh up a reproach against his neighbour.

Westminster Project - Francis Beattie on the Second Table of the Law as a means of grace