Yours, Etc.: Origins and Uses of 8 Sign-Offs
Old English osanna, via Latin and Greek from Hebrew hosha'na, probably a shortening of hoshi'ah-nna "save, we pray" (cf. Psalms cxviii:25), from imperative of y-sh- (cf. yeshua "salvation, deliverance, welfare") + emphatic particle -na. Originally an appeal for deliverance; used in Christian Church as an ascription of praise, because when Jesus entered Jerusalem this was shouted by Galilean pilgrims in recognition of his messiahhood (Matt. xxi:9, 15, etc.).
Save now! or Save, we beseech, (Matt. 21:9). This was a customary form of acclamation at the feast of Tabernacles. (Comp. Ps. 118:25.)