His mouth is full of cursing and deceit and fraud: under his tongue is mischief and vanity.
His mouth is full of cursing. The sinner's guilt toward his neighbour, wherein he is confirmed by prosperity, is set forth in respect to his words first; Psalms 10:8-10 pass on to his deeds. His 'cursings' are such as he utters upon himself in perjury, in order that he may get possession of his neighbour's goods. Compare Psalms 59:12, where, as here, cursing is connected with lying or deceit.
And fraud. Gesenius translates х tok (H8496)], 'oppression.' The usual meaning is as the English version. The Hebrew root [taawak] means 'middle;' i:e., something hidden in the middle of the heart: the tongue speaking one thing, while the heart thinks another. The Septuagint read 'bitterness' х maarowt (H4796)] instead of "deceit" х mirmowt (H4820)].
Under his tongue. The metaphor is from serpents, whose venom is hidden in little bags under the teeth, and from thence is pressed out at will (Psalms 140:3). Instead of saying, 'upon the tongue,' the Psalmist says "under" it, to imply that the prosperous sinner has a whole storehouse of evil "under his tongue," from which, as need may require, he takes a part, to lay upon his tongue in speaking. Hence, the sinner's mouth is said to be "full of cursing."
Mischief and vanity, х `aamaal (H5999) waa'aawen (H205)] - 'sorrow and mischief.' The former means literally, labour, then the hardship flowing from it: so sorrow in general. The latter means vanity, or mischievous iniquity, which, in the view of Scripture, is the height of vanity. His mouth is like a magazine of sorrow and mischief (Job 20:12; Song of Solomon 4:11).