Keep your tongue from evil, and your lips from speaking deceit.
Read Chapter 34
Augustine of Hippo
17. Let not a Christian then murmur, let him see whose steps he followeth: but if he loveth good days, let him hearken unto Him teaching and saying, "Come, ye children, hearken unto Me; I will teach you the fear of the Lord." What wouldest thou? Life and good days. Hear, and do. "Keep thy tongue from evil" (ver. 13). This do. I will not, saith a miserable man, I will not keep my tongue from evil, and yet I desire life and good days. If a workman of thine should say to thee, I indeed lay waste this vineyard, yet I require of thee my reward; thou broughtest me to the vineyard to lop and prune it, I cut away all the useful wood, I will cut short also the very trunks of the vines, that thou have thereon nothing to gather, and when I have done this, thou shall repay to me my labour. Wouldest thou not call him mad? Wouldest thou not drive him from thy house or ever he put his hand to the knife? Such are those men who would both do evil, and swear falsely, and speak blasphemy against God, and...
Guile. He very properly begins with regulating the tongue, as this member may prove very dangerous, Proverbs xviii. 21., and James iii. 5. By detraction, it wounds three people; and it causes no less evil by flattery. (Calmet)