A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
pro 21:1(Pro. 21:1-31)
rivers--irrigating channels (Psa 1:3), whose course was easily turned (compare Deu 11:10). God disposes even kings as He pleases (Pro 16:9; Psa 33:15).
pro 21:2(Compare Pro 14:2; Pro. 16:2-25).
pro 21:3(Compare Psa 50:7-15; Isa 1:11, Isa 1:17).
pro 21:4high look--(Compare Margin; Psa 131:1).
proud heart--or, "heart of breadth," one that is swollen (compare Psa 101:5).
ploughing--better "lamp," a frequent figure for prosperity (Pro 20:20); hence joy or delight.
pro 21:5The contrast is between steady industry and rashness (compare Pro 19:2).
pro 21:6The getting--or, "what is obtained" (compare Job 7:2; Jer 22:13, Hebrew).
vanity . . . to and fro--as fleeting as chaff or stubble in the wind (compare Pro 20:17-21; Psa 62:10). Such gettings are unsatisfactory.
them . . . death--act as if they did (Pro 8:36; Pro 17:19).
pro 21:7robbery--or, "destruction," especially oppression, of which they are authors.
shall destroy--literally, "cut with a saw" (Kg1 7:9), that is, utterly ruin them. Their sins shall be visited on them in kind.
to do judgment--what is just and right.
pro 21:8of man--any one; his way is opposed to truth, and also estranged from it. The pure proves himself such by his right conduct.
pro 21:9corner--a turret or arbor on the roof.
wide house--literally, "house of fellowship," large enough for several families.
pro 21:10So strongly does he desire to do evil (Psa 10:3; Ecc 8:11), that he will not even spare his friend if in his way.
pro 21:11(Compare Pro 19:25). That which the simple learn by the terrors of punishment, the wise learn by teaching.
pro 21:12(Compare Psa 37:35-38; Psa 73:17, Psa 73:20).
house--family or interests.
overthroweth--either supply "God" (compare Pro 10:24), or the word is used impersonally.
pro 21:13The principles of retribution, often taught (compare Psa 18:26; Mat 7:1-12).
pro 21:14The effect of bribery (Pro 17:23) is enhanced by secrecy, as the bribed person does not wish his motives made known.
pro 21:15But the just love right and need no bribes. The wicked at last meet destruction, though for a time happy in concealing corruption.
pro 21:16the way of understanding--(Compare Pro 12:26; Pro 14:22).
remain--that is, rest as at a journey's end; death will be his unchanging home.
pro 21:17Costly luxuries impoverish.
pro 21:18(Compare Pro 11:8). By suffering what they had devised for the righteous, or brought on them, the wicked became their ransom, in the usual sense of substitutes (compare Jos 7:26; Est 7:9).
pro 21:19(Compare Pro 21:9).
wilderness--pasture, though uninhabitable ground (Psa 65:12).
pro 21:20The wise, by diligence and care, lay up and increase wealth, while fools
spend--literally, "swallow it up," greedily.
pro 21:21He who tries to act justly and kindly (Psa 34:14) will prosper and obtain justice and honor.
pro 21:22"Wisdom is better than strength" (Ecc 7:19; Ecc 9:15).
strength . . . thereof--that in which they confide.
pro 21:23(Compare Pro 13:2-3; Jam 3:6-10).
pro 21:24The reproachful name is deserved by those who treat others with anger and contempt.
pro 21:25desire--that is, of ease and idleness brings him to starvation.
pro 21:26The sin of covetousness marks the sluggard, as the virtue of benevolence the righteous.
pro 21:27God regards the heart, and hypocrisy is more odious than open inconsistency.
wicked mind--or, "design" (Pro 1:4).
pro 21:28(Compare Pro 19:5).
that heareth--or heeds instruction, and so grows wise.
speaketh constantly--or sincerely (compare Hab 1:5), and hence is believed (Pro 12:19; Jam 1:19).
pro 21:29hardeneth his face--is obstinate.
directeth . . . way--considers it, and acts advisedly.
pro 21:30Men's best devices and reliances are vain compared with God's, or without His aid (Pro 19:21; Psa 20:7; Psa 33:17).