What does Psalm 35:21 mean?

David's enemies lied and invented false accusations (Psalm 35:11–16). They claimed to have seen him commit a crime or moral failure. The word translated "aha!" is he'āh' and is used for emphasis (Psalm 35:25), much like a modern person making a sarcastic laughing sound. In this case, the liars who attack David say something to the effect of "Hah! We saw him do it!" Of course, they saw no such thing.

These wicked slanderers resemble the false witnesses who testified against Jesus. They, too, were deceitful and depraved (Proverbs 6:16–19). Those who make up lies to harm others follow the lead of the Devil (Exodus 20:16). The religious leaders who plotted to kill Jesus thought of themselves as children of Abraham, founder of the Hebrew people. Jesus corrected them, saying, "You are of your father the devil, and your will is to do your father's desires. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he lies, he speaks out of his own character, for he is a liar and the father of lies" (John 8:44).

Revelation 21:8 places "liar" in the category of sins which marks an unbeliever. A person whose life is characterized by dishonesty or deception—and especially with slander of others—proves they live in opposition to God and His will. The same fate is ascribed to those defined as cowardly, the faithless, the detestable, murderers, the sexually immoral, sorcerers, and idolaters; all of whom will be in the lake of fire.