John Calvin Commentary Psalms 109:2

"For the mouth of the wicked and the mouth of deceit have they opened against me: They have spoken unto me with a lying tongue." — Psalms 109:2 (ASV)

Because the mouth of the wicked—David here very plainly declares that he was all the more anxious to obtain help from God because justice was not found among men. And though it is probable that he was rashly and furiously assailed, nevertheless, he complains that the mouth of deceit and fraud had been opened against him and that he was surrounded with false tongues. Therefore, to those who were ignorant of his real situation, there would appear to be some plausible pretext for his being loaded with reproaches, so much so, indeed, that he would not be able to evade the charge of criminality.