"Thou shalt not bear false witness against thy neighbor." (Exodus 20:16)
"But he, willing to justify himself, said unto Jesus, And who is my neighbor?" (Luke 10:29)
Everyone, that's who.
God, in the former verse, gives Moses a command against lying that covers so much more than the mere telling of non-truths. To "bear false witness" also refers to the way we represent ourselves to others, and when you hear Jesus say "love thy neighbor as thyself", it also adds depth to the commandment. It's one thing to lie to someone else and another thing to lie to yourself. Both instances ultimately harm Jesus because, "as ye have done it unto one of the least of these my brethren, ye have done it unto me." (Matthew 25:40)
"A man that beareth false witness against his neighbour is a maul, and a sword, and a sharp arrow." (Proverbs 25:18)
Ouch. That's pretty blunt. When we are dishonest with ourselves, with others and with God, we cause harm. And when we think no one sees the lies we speak, we're only digging ourselves in deeper. Think about the imperative that we as Christians have. To be up front, honest and transparent to the world we serve as unto Jesus. If the Gospel we preach isn't backed with forthrightness and truth, those we're trying to reach may never have another reason to come to God. If you or I represent the Lord and we're not allowing the Holy Spirit (the "Spirit of truth", see John 14:17) to shine through us because of our transparency toward God, then I can see how Solomon would refer to the "false witness" as a weapon. As something doing more harm than good.
Consider this verse from the Psalms (58:3), "The wicked are estranged from the womb: they go astray as soon as they be born, speaking lies." Nothing to worry about here. Listen: "if thou shalt confess with thy mouth the Lord Jesus and shalt believe in thine heart that God hath raised Him from the dead, thou shalt be saved." (Romans 10:9) This is what happens. We get reborn. So even if you were "estranged from the womb...speaking lies", it has no bearing on who you are now, as a believer in Christ Jesus. No pathology can touch you because you have the "mind of Christ" (see 1 Corinthians 2:16).