11 What does he mean to express by the “thigh”? The flesh. Whence those words, “A prince shall not depart from Judah; and a lawgiver from his thighs”? Did not Abraham himself (to whom was promised the seed in which “all the nations of the earth were to be blessed”), when he sent his servant to seek and to bring home a wife for his son, being by faith fully persuaded, that in that, so to speak, contemptible seed was contained the great Name; that is, that the Son of God was to come of the seed of Abraham, out of all the children of men; did not he, I say, cause his servant to swear unto him in this manner, saying, “Put your hand under my thigh,” and so swear; as if he had said, “Put your hand on the altar, or on the Gospel, or on the Prophet, or on any holy thing.”
“Put” (he says) “your hand under my thigh;” having full confidence, not ashamed of it as unseemly, but understanding therein a truth. “With Your beauty and Your glory.” Take to You that righteousness, in which You are at all times beautiful and glorious. “And speed on, and proceed prosperously, and reign”. Do we not see it so? Is it not already come to pass? He has “sped on; has proceeded prosperously, and He reigns;” all nations are subdued unto Him. What a thing was it to see that “in the Spirit,” of which same thing it is now in our power to experience in the reality!
At the time when these words were said, Christ did not yet “reign” thus; had not yet sped on, nor “proceeded prosperously.” They were then being preached, they have now been fulfilled: in many things we have God's promise fulfilled already; in some few we have to claim its fulfilment yet.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)