2 For the Gospel, the reading of which you have just been listening to, says: “On the next day much people that had come to the feast, when they heard that Jesus was coming to Jerusalem, took branches of palm trees and went forth to meet Him, and cried, Hosanna: blessed is He that comes in the name of the Lord as the King of Israel.” The branches of palm trees are laudatory emblems, significant of victory, because the Lord was about to overcome death by dying, and by the trophy of His cross to triumph over the devil, the prince of death. The exclamation used by the worshipping people is Hosanna, indicating, as some who know the Hebrew language affirm, rather a state of mind than having any positive significance; just as in our own tongue we have what are called interjections, as when in our grief we say, Alas! Or in our joy, Ha! Or in our admiration, O how fine! Where O! expresses only the feeling of the admirer. Of the same class must we believe this word to be, as it has failed to find an interpretation both in Greek and Latin, like that other, “Whosoever shall say to his brother, Raca.” For this also is allowed to be an interjection, expressive of angry feelings.
Source: Tractates on the Gospel of John (New Advent)