He deviseth mischief - He plots schemes and plans to bring it to pass.
He soweth discord - Between men and their wives, by seducing the latter from their fidelity. See the preceding quotation.

Frowardness is in his heart,.... Or perverse things; evil habits and principles of sin; all manner of wickedness, errors and heresies; things contrary to right reason, repugnant to the will and law of God, and the reverse of sound doctrine; all evil thoughts and evil things; see Matthew 15:19;
he deviseth mischief continually; against his neighbours, and especially against good men; he is continually planning schemes, contriving methods, ways, and means, how to disturb, distress, and ruin men; being a true child of Belial, or of the devil, his heart is the forge where he is continually framing wickedness in one shape or another; and the ground which he is always ploughing up and labouring at to bring forth sin and wickedness, and with which it is fruitful;
he soweth discord; or "strifes" (z): the Syriac version adds, "between two"; which Jarchi interprets between a man and his Maker; rather between a man and his neighbour; between one friend and another; between husband and wife, parents and children, brethren and brethren, magistrates and subjects; between kings and princes of the earth in which sort of work the man of sin, antichrist, has been very busy. The Targum is, "he casteth out strifes", as firebrands among men. The words in the Hebrew text are, "he sendeth out discord", or "strife" (a); these are the messengers sent out by him to make mischief.
(z) "contentiones", Pagninus, Montanus, Junius & Tremellius, Piscator, Mercerus, Gejerus; "jurgia", V. L. "lites", Baynus, Cocceius; "litigia", Schultens. (a) "mittet", Pagninus, Montanus; "immittit", Junius & Tremellius, Piscator, Mercerus, Gejerus, Schultens, Michaelis.

Frowardness--as in Proverbs 2:14.
deviseth--literally, "constructs, as an artisan."
mischief--evil to others.
discord--especially litigation. Cunning is the talent of the weak and lazy.

In this verse is continued the description of the subject, only once returning to the particip. The clauses are arranged independently, but logically according to the complex conception of the subject. תּהפּכות .tce are just the knaveries, i.e., the malicious wickedness which comes to light in word and deportment as עקשׁות פה. Regarding the double figure of the smithy and of agriculture underlying חרשׁ, machinari, vid., at Proverbs 3:29, and regarding the omission of the הוּא to חרשׁ, at Psalm 7:10. The phrase שׁלּח מדנים (as Proverbs 6:19, Proverbs 16:28), to let loose disputes, so that they break forth, reminds us rather of the unfettering of the winds by Aeolus than of the casting in of the apple of discord. Instead of מדנים the Kerı̂ has מדינים; on the other hand, מדנים remains uncorrected Proverbs 6:19; Proverbs 10:12. The form מדינים occurs once, Proverbs 18:18, and its constr. מדיני once, Proverbs 19:13. Everywhere else the text has מדונים, for which the Kerı̂ has מדינים, Proverbs 18:19; Proverbs 21:9, Proverbs 21:19; Proverbs 23:29; Proverbs 25:24; Proverbs 26:21; Proverbs 27:15. The forms מדין and מדן are also recognised: the former stands alone without any analogous example; the latter is compared at least with מצד, Arab. masâd (Psalmen, p. 163, 3). Probably these two forms are warranted by Genesis 25:2, cf. Genesis 37:28, Genesis 37:36, where מדין and מדן occur as the names of two sons of Abraham by Keturah. But the national name מדינים is no reason for the seven times laying aside of the regular form מדונים, i.e., מדונים, which is the plur. of מדון after the forms מאורים, מעורים, although מדוּנים, after the forms מבוּשׁים, מצוּקים, is also found.