My Lord. The Hebrew noun denotes a respected or superior person. It is used as a title in addressing kings and other dignitaries (Gn 19:2; 23:6; 1 Sm 22:12; 2 Sm 13:32). The king to or "concerning" whom this prophetic oracle speaks is by that very word of God made to be the "King of kings and Lord of lords" (Rv 19:16). His elevation to the right hand of power is to coincide with the defeat of enemies, reduced to serve as a footstool (Jos 10:24; 1 K 5:3). For the fulfillment of this promise see Acts 2:25; 1 Co 15:25; Heb 1:13; 10:13."
Martin Franzmann, Concordia Self-study Commentary [commentary on Psalms], p. 395-396