The Lord says to my Lord: “Sit at my right hand until I make your enemies a footstool for your feet” (NIV)
We’ll defer here to a very helpful comment from the Biblical Unitarian website:
Trinitarian commentators frequently argue that “my Lord” in this verse is the Hebrew word adonai, another name for God, and is therefore proof of the divinity of the Messiah. But not only is this not a valid argument, this verse is actually one of the great proofs of the complete humanity of the promised Messiah. The Hebrew word translated “my lord” is adoni (pronounced “Adon nee” [1] ) in the standard Hebrew texts. This word is always used in Scripture to describe human masters and lords, but never God. Unfortunately, most Hebrew concordances and lexicons give only root words, not the word that actually occurs in the Hebrew text. This is one reason why biblical research done by people using only tools such as a Strong’s Concordance will often be limited. [2] While this usually does not affect the interpretation of the text, sometimes it makes a great deal of difference, such as in Psalm 110:1