Blessed is the coming kingdom of our father David. Hosanna in the highest. And he entered Jerusalem into the temple, etc. We read in the Gospel of John that the crowds, having been fed with five loaves and two fishes, wanted to seize Jesus and make him king; but to prevent this from happening, he fled to the mountain and prayed. Now, however, as he comes to Jerusalem to suffer, he does not avoid those who make him king, who lead him to the royal city with a glorious procession and hymns worthy of the Son of God and king; he does not suppress the voices of those who proclaim the restoration of the kingdom of patriarch David in him and the recovery of the blessings of old. Why, then, does he now willingly embrace what he avoided before by fleeing, and not refuse to accept the kingdom, which while still living in the world he did not wish to receive, but now, about to depart from the world through the passion of the cross, he does not refuse to accept, unless it is to openly teach that he is a king not of a temporal and earthly, but of an eternal empire in heaven? To which kingdom indeed he would arrive through the contempt of death, the glory of resurrection, and the triumph of ascension. Hence it is that after the resurrection, appearing to the disciples, he said: All power in heaven and on earth has been given to me, and other things of the same place. It should certainly be noted how great a consonance there is between the crowd praising the Lord and the voice of the archangel Gabriel announcing to the virgin mother, who said: He will be great and will be called the Son of the Most High. And the Lord God will give him the throne of David his father, and he will reign over the house of Jacob forever (Luke I). Now, the Lord took the seat or kingdom of David to call the people, to whom David once provided the reins of the temporal kingdom as well as examples of justice, and whom they were accustomed to inspire with the modulations of spiritual hymns to faith and love for their Creator, to the heavenly and immortal kingdom by means of words, gifts, deeds, and promises worthy of such a mediator between God and men, and to lead them to the very vision of God the Father. In this it is joined: Hosanna in the highest, that is, salvation. This clearly shows that the coming of Christ is not only the salvation of men but of the whole world; uniting the earthly with the heavenly, so that every knee shall bow to him, of those in heaven, on earth, and under the earth. It should certainly be noted that the word Hosanna, a Hebrew word, is composed of two parts, corrupted and intact. For save or save us is said by them Osi, while anna is an interjection of one who is beseeching. Just as among the Latins there is an interjection for one who is in pain, "heu," and an interjection for one who is marveling, "papae." Finally, in the hundred and seventeenth psalm, where the seventy interpreters translated "O Lord, save me," in Hebrew it is written: Anna Adonai, osi anna. This our interpreter Jerome, elucidating more diligently, translated thus: I beseech you, O Lord, save I beseech. For it signifies the same thing, "O Lord," through the interjection of beseeching, as "I beseech you, O Lord," through the very word of supplication. Hosanna thus signifies "save, I beseech," with the vowel letter i, which terminates the first word when it is perfectly said osi, being consumed by the force of the vowel letter a, with which the following word begins anna. Which the metrical poets call synaloepha in scanning verses, although when scanning they leap over the written letter; however, in this word Hosanna, the letter i is neither even written, but with the sense of the speakers preserved, is utterly extinguished.
Source: On the Gospel of Mark