Adam Clarke on Psalms 118:25

Save now, I beseech thee - These words were sung by the Jews on the feast of tabernacles, when carrying green branches in their hands; and from the הושיעה נא hoshiah nna, we have the word hosanna. This was sung by the Jewish children when Christ made his public entry into Jerusalem. See Mat 21:9 (note), and see the note there, in which the word and the circumstance are both explained.