A deceptive witness = telling misleading, incomplete, or untrue stories about yourself or someone else.
God is pretty big on witnesses to corroborate things. He particularly comments on this broadly in Deuteronomy.
“A single witness shall not rise up against a man on account of any iniquity or any sin which he has committed; on the evidence of two or three witnesses a matter shall be confirmed.” – Deuteronomy 19:15
Jesus Himself refers to the need for witnesses as we hold one another accountable for how we treat others.
“If your brother sins, go and show him his fault in private; if he listens to you, you have won your brother. But if he does not listen to you, take one or two more with you, so that by the mouth of two or three witnesses every fact may be confirmed. If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. Truly I say to you, whatever you bind on earth shall have been bound in heaven; and whatever you loose on earth shall have been loosed in heaven.”
– Matthew 18:15-18
We live in a world culture who believes “if I simply say it – it is true.” The results of this are devastating to the soul. Inherent in the God Story is the fact that we cannot trust ourselves, so we must submit ourselves to one another in order to have a healthy society. This is why God gave His people His Law. And why He introduced spiritual and social and moral accountability.
When we freely give false witness (lie) about others without accountability we do harm to one another. Countering this propensity begins with truth between myself and God, and then a clear, true conscience toward others. This is a free way to live, and avoids the bondage of what occurs when we intentionally speak falsely about our neighbor. As well as how we rob others regarding reputation etc. when we do such things. Sobering.