Albert Barnes Commentary Job 31:30

"(Yea, I have not suffered by mouth to sin By asking his life with a curse);" — Job 31:30 (ASV)

Neither have I allowed my mouth - Margin, as in Hebrew, "palate." The word is often used for the mouth in general, and especially as the organ of voice, due to the use and importance of the palate in speaking (Proverbs 8:7). For my palate (חכי chikiy) speaketh truth. It is used as the organ of taste (Job 12:11; Psalms 119:103).

By wishing a curse to his soul - It must have been an extraordinary degree of piety that would allow a man to say this truthfully: that he had never harbored a wish for injury toward an enemy. Few are the people, probably, even now, who could say this and who are able to keep their minds free from every wish that calamities and woes might overtake those who are seeking to harm them. Yet this is the nature of true religion. It controls the heart, represses the angry and revengeful feelings, and creates in the soul an earnest desire for the happiness even of those who injure us.