Albert Barnes Commentary Psalms 140:9

"As for the head of those that compass me about, Let the mischief of their own lips cover them." — Psalms 140:9 (ASV)

As for the head of those that compass me about - Luther renders this, “The calamity which my enemies design against me must fall upon their own heads.” The passage stands in contrast with Psalms 140:7: You have covered my head, etc.

As for his own head, it had been protected in the day of battle. In reference now to the heads of his enemies—of those who surrounded him—he prays that what they had designed for “his” head might come by a just retribution on their own.

The phrase “compass me about” refers to his enemies as being numerous and as surrounding him on every side. See Psalms 40:12; Psalms 88:17; Psalms 109:3; Psalms 118:10–12.

Let the mischief of their own lips cover them - Come upon them. The mischief which they have designed against me; that which they have conspired to bring on me. The reference is to a combination against him, or to some agreement they had made to destroy him.