Matthew 22:41-46

Now while the Pharisees were gathered together, Jesus asked them a question, saying, “What do you think about the Christ? Whose son is he?” They said to him, “The son of David.” He said to them, “How is it then that David, in the Spirit, calls him Lord, saying, “‘The Lord said to my Lord, “Sit at my right hand, until I put your enemies under your feet”’? If then David calls him Lord, how is he his son?” And no one was able to answer him a word, nor from that day did anyone dare to ask him any more questions.

In the Spirit - inspired by the Holy Spirit and, therefore, true and accurate.

The Lord was referring to Psalm 110 (vs.43-44), which the Jews recognized as Messianic.

Lord (v.44, 1st use) = Jehovah

Lord (v.44, 2nd use) = Master

The Lord said to my Lord - "Christ was trying to make the Pharisees see that the Son of David was also the Lord of David (Ps 110:1); i.e., the Messiah was both David's human descendant and divine Lord." (Ryrie) 

"The LORD says to my Lord: Sit at My right hand until I make Your enemies a footstool for Your feet (Psalm 110:1). This psalm was universally recognized as messianic. The One invited to sit at the Lord’s right hand was the Messiah. The “LORD” who invited Him to sit at His right hand was the God of Abraham. The Messiah was referred to as “my Lord.” With this interpretation the Pharisees would have been in agreement. Christ addressed this question to them: If the Messiah was the “son,” or descendant, of David, “How is it then that David, speaking by the Spirit, called him ‘Lord’?” It was not natural for one to call his own son “my Lord.” The fact that Messiah was David’s Son testified to Messiah’s true humanity, but the fact that David called Him “my Lord” testified to His true and undiminished deity, for Lord was a title for Deity. Christ interrogated the Pharisees again, Asking, “If then David calls Him ‘Lord,’ how can He be his son?” (v.45). The Psalm taught the true humanity and the true deity of Messiah. It was just such a claim as the psalmist foretold of Messiah that Jesus made for Himself. If the Pharisees answered that David called Him his Lord because He is God, then they could not object to Christ, David’s Son according to the flesh, claiming to be the Son of God. If they agreed that Messiah was to be truly human and truly God, they must ceases their objections to Christ’s claim concerning His person. The Pharisees realized the dilemma that faced them and refused to answer." (Pentecost)