I hesitated making this an answer because I am simply reposting from another site, verbatim, but as that site seems not to be working (and I had to lift this from the google chache) so I can't simply put the link as a comment, I am pasting it here. Apologies to anyone who finds that my answer isn't so much my answer as another's:
...the L--rd says to my lord: "Sit at My right hand, until I make your enemies your footstool."
Here David is repeating G--d's promise concerning Abraham's victory over the four kingdoms (Genesis chapter 14). After this war, Melchizedek hailed the returning hero and blessed him:
And Melchizedek, king of Salem, brought forth bread and wine; and he was a priest of the Supreme G--d. And he blessed him, and said: "Blessed be Abram by the Supreme G--d, Maker of heaven and earth; and blessed be the Supreme G--d, Who has delivered your enemies into your hand."
David alludes to this blessing later in the Psalm under discussion:
The L--rd has sworn, and will not recant: "You [Abraham] are a priest forever, after the manner of Melchizedek."
Referring specifically to Abraham's miraculous victory, he declares:
The L--rd is at your right hand, He has shattered119 kings on the day of His wrath.
To make it appear as if G--d is here promising that Jesus' enemies will be conquered, the authors of the New Testament boldly misquoted our verse under discussion to:
The L--rd said to my Lord....
Thus, giving the impression that one Divine Being (G--d) is here promising another "divine" being (Jesus) that his enemies will be conquered!
In Hebrew, unlike in English, there are two separate words for "L--rd" and "lord": A--noy and adoni (literally, "my lord"). The latter appears in the following verses:
Hear us, my lord [Abraham]; you are a mighty prince among us....
I Kings 1:2
Therefore his servants said to [David]: "Let there be sought for my lord, the king, a young virgin...."
I Kings 1:17
And she said to [David]: "My lord, you swore by the L--rd, your G--d, to your handmaid: `Surely Solomon your son shall reign after me, and he shall sit upon my throne.' "
from "Their Hollow Inheritance" by M. Drazin.