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
psa 109:1The writer complains of his virulent enemies, on whom he imprecates God's righteous punishment, and to a prayer for a divine interposition in his behalf appends the expression of his confidence and a promise of his praises. This Psalm is remarkable for the number and severity of its imprecations. Its evident typical character (compare Psa 109:8) justifies the explanation of these already given, that as the language of David respecting his own enemies, or those of Christ, it has respect not to the penitent, but to the impenitent and implacable foes of good men, and of God and His cause, whose inevitable fate is thus indicated by inspired authority. (Psa. 109:1-31)
God of my praise--its object, thus recognizing God as a certain helper. Be not silent (compare Psa 17:13; Psa 28:1).
psa 109:2For the mouth . . . opened--or, "They have opened a wicked mouth"
against me--literally, "with me," that is, Their intercourse is lying, or, they slander me to my face (Mat 26:59).
psa 109:3(Compare Psa 35:7; Psa 69:4).
psa 109:4They return evil for good (compare Psa 27:12; Pro 17:13).
I give myself unto prayer--or literally, "I (am) prayer," or, "as for me, prayer," that is, it is my resource for comfort in distress.
psa 109:6over him--one of his enemies prominent in malignity (Psa 55:12).
let Satan stand--as an accuser, whose place was the right hand of the accused (Zac 3:1-2).
psa 109:7The condemnation is aggravated when prayer for relief is treated as a sin.
psa 109:8The opposite blessing is long life (Psa 91:16; Pro 3:2). The last clause is quoted as to Judas by Peter (Act 1:20).
office--literally, "charge," Septuagint, and Peter, "oversight" [Pe1 5:2].
psa 109:9Let his family share the punishment, his children be as wandering beggars to prowl in their desolate homes, a greedy and relentless creditor grasp his substance, his labor, or the fruit of it, enure to strangers and not his heirs, and his unprotected, fatherless children fall in want, so that his posterity shall utterly fail.
psa 109:13posterity--literally, "end," as in Psa 37:38, or, what comes after; that is, reward, or success, or its expectation, of which posterity was to a Jew a prominent part.
psa 109:14Let the iniquity of his fathers be remembered, &c.--Added to the terrible overthrow following his own sin, let there be the imputation of his parents' guilt, that it may now come before God, for His meting out its full consequences, in cutting off the memory of them (that is, the parents) from the earth (Psa 34:16).
psa 109:16Let God remember guilt, because he (the wicked) did not remember mercy.
poor and needy . . . broken in heart--that is, pious sufferer (Psa 34:18; Psa 35:10; Psa 40:17).
psa 109:17Let his loved sin, cursing, come upon him in punishment (Psa 35:8), thoroughly fill him as water and oil, permeating to every part of his system (compare Num 5:22-27), and become a garment and a girdle for a perpetual dress.
psa 109:20Let this . . . reward--or, "wages," pay for labor, the fruit of the enemy's wickedness.
from the Lord--as His judicial act.
psa 109:21do . . . for me--that is, kindness.
wounded--literally, "pierced" (Psa 69:16, Psa 69:29).
psa 109:23like the shadow--(Compare Psa 102:11).
tossed up and down--or, "driven" (Exo 10:19).
psa 109:24Taunts and reproaches aggravate his afflicted and feeble state (Psa 22:6-7).
psa 109:26Let my deliverance glorify Thee (compare Psa 59:13).
psa 109:28In confidence that God's blessing would come on him, and confusion and shame on his enemies (Psa 73:13), he ceases to regard their curses, and anticipates a season of joyful and public thanksgiving; for God is near to protect (Psa 16:8; Psa 34:6) the poor from all unrighteous judges who may condemn him.