What does Psalm 59:12 mean?

The incident which inspired this psalm was an attempted assassination. Saul, the king, sent his men to watch David's house. Michal, who was David's wife and Saul's daughter, helped David escape the plot (1 Samuel 19:11–16). David has already prayed for rescue from such men (Psalm 59:1–2). He has noted that they wait in ambush (Psalm 59:3) and compared them to unclean animals (Psalm 59:6–7). He prays for God to make an example of these evil ones (Psalm 59:11) before they are utterly destroyed (Psalm 59:13).

Now David includes his enemies' evil words as a reason for them to be judged. To arrange this attempted murder, Saul and his men would have plotted and discussed their plans. David knows that Saul is targeting him for no reason (Psalm 59:3; 1 Samuel 20:30–33). To justify a murder plot, lies must be involved. What they say is evil, and prideful, presuming they can invent reasons to attack the innocent and not suffer consequences.

Proverbs 6:16–19 includes sins related to words among those God intensely hates. In that passage, those are labeled as "a lying tongue" and "a false witness who breathes out lies." In the days of the early church, Ananias and Saphira lied to the Holy Spirit. This brought God's judgment in the form of death (Acts 5:1–11).