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 110:1The explicit application of this Psalm to our Saviour, by Him (Mat 22:42-45) and by the apostles (Act 2:34; Co1 15:25; Heb 1:13), and their frequent reference to its language and purport (Eph 1:20-22; Phi 2:9-11; Heb 10:12-13), leave no doubt of its purely prophetic character. Not only was there nothing in the position or character, personal or official, of David or any other descendant, to justify a reference to either, but utter severance from the royal office of all priestly functions (so clearly assigned the subject of this Psalm) positively forbids such a reference. The Psalm celebrates the exaltation of Christ to the throne of an eternal and increasing kingdom, and a perpetual priesthood (Zac 6:13), involving the subjugation of His enemies and the multiplication of His subjects, and rendered infallibly certain by the word and oath of Almighty God. (Psa 110:1-7)
The Lord said--literally, "A saying of the Lord," (compare Psa 36:1), a formula, used in prophetic or other solemn or express declarations.
my Lord--That the Jews understood this term to denote the Messiah their traditions show, and Christ's mode of arguing on such an assumption (Mat 22:44) also proves.
Sit . . . at my right hand--not only a mark of honor (Kg1 2:19), but also implied participation of power (Psa 45:9; Mar 16:19; Eph 1:20).
Sit--as a king (Psa 29:10), though the position rather than posture is intimated (compare Act 7:55-56).
until I make, &c.--The dominion of Christ over His enemies, as commissioned by God, and entrusted with all power (Mat 28:18) for their subjugation, will assuredly be established (Co1 15:24-28). This is neither His government as God, nor that which, as the incarnate Saviour, He exercises over His people, of whom He will ever be Head.
thine enemies thy footstool--an expression taken from the custom of Eastern conquerors (compare Jos 10:24; Jdg 1:7) to signify a complete subjection.
psa 110:2the rod of thy strength--the rod of correction (Isa 9:4; Isa 10:15; Jer 48:12), by which Thy strength will be known. This is His Word of truth (Isa 2:3; Isa 11:4), converting some and confounding others (compare Th2 2:8).
out of Zion--or, the Church, in which God dwells by His Spirit, as once by a visible symbol in the tabernacle on Zion (compare Psa 2:6).
rule thou, &c.--over enemies now conquered.
in the midst--once set upon, as by ferocious beasts (Psa 22:16), now humbly, though reluctantly, confessed as Lord (Phi 2:10-11).
psa 110:3Thy people . . . willing--literally, "Thy people (are) free will offerings"; for such is the proper rendering of the word "willing," which is a plural noun, and not an adjective (compare Exo 25:2; Psa 54:6), also a similar form (Jdg 5:2-9).
in the day of thy power--Thy people freely offer themselves (Rom 12:1) in Thy service, enlisting under Thy banner.
in the beauties of holiness--either as in Psa 29:2, the loveliness of a spiritual worship, of which the temple service, in all its material splendors, was but a type; or more probably, the appearance of the worshippers, who, in this spiritual kingdom, are a nation of kings and priests (Pe1 2:9; Rev 1:5), attending this Priest and King, clothed in those eminent graces which the beautiful vestments of the Aaronic priests (Lev 16:4) typified. The last very obscure clause--
from the womb . . . youth--may, according to this view, be thus explained: The word "youth" denotes a period of life distinguished for strength and activity (compare Ecc 11:9) --the "dew" is a constant emblem of whatever is refreshing and strengthening (Pro 19:12; Hos 14:5). The Messiah, then, as leading His people, is represented as continually in the vigor of youth, refreshed and strengthened by the early dew of God's grace and Spirit. Thus the phrase corresponds as a member of a parallelism with "the day of thy power" in the first clause. "In the beauties of holiness" belongs to this latter clause, corresponding to "Thy people" in the first, and the colon after "morning" is omitted. Others prefer: Thy youth, or youthful vigor, or body, shall be constantly refreshed by successive accessions of people as dew from the early morning; and this accords with the New Testament idea that the Church is Christ's body (compare Mic 5:7).
psa 110:4The perpetuity of the priesthood, here asserted on God's oath, corresponds with that of the kingly office just explained.
after the order-- (Heb 7:15) after the similitude of Melchisedek, is fully expounded by Paul, to denote not only perpetuity, appointment of God, and a royal priesthood, but also the absence of priestly descent and succession, and superiority to the Aaronic order.
psa 110:5at thy right hand--as Psa 109:31, upholding and aiding, which is not inconsistent with Psa 110:1, where the figure denotes participation of power, for here He is presented in another aspect, as a warrior going against enemies, and sustained by God.
strike through--smite or crush.
kings--not common men, but their rulers, and so all under them (Psa 2:2, Psa 2:10).
psa 110:6The person is again changed. The Messiah's conquests are described, though His work and God's are the same. As after a battle, whose field is strewn with corpses, the conqueror ascends the seat of empire, so shall He "judge," or "rule," among many nations, and subdue
the head--or (as used collectively for "many") "the heads," over many lands.
wound--literally, "smite," or "crush" (compare Psa 110:5).
psa 110:7As a conqueror, "faint, yet pursuing" [Jdg 8:4], He shall be refreshed by the brook in the way, and pursue to completion His divine and glorious triumphs.