Hosanna in the Highest

What does Hosanna mean? In the Old Testament 'it is used only in verses such as "help" or "save, I pray"', but in the Gospels 'it is used as a shout of jubilation.' Wikipedia links a blurry scan as an example of the complicated effort at reconciliation of meanings. A Jewish site is easier to read. That it did shift meaning is clear enough from the rituals associated with Hoshana Rabbah (the seven hoshanot in honor of patriarch or prophet).

It isn't hard for words to shift meaning: hussy changed from housewife to "improper woman" in a little over a century. You'd think that liturgical words would be more stable, but "Thou" is widely believed to be a holy and respectful way to address God, and its original intimate familiar meaning got lost. If a phrase becomes less popular in everyday language, it would be easy to pick up new connotations, and have those eventually become the denotation.

If no one can see God and live, and if the angels Ezekiel saw had to hide their faces, perhaps the closer you get to God's glory the more you need His protection: joy and fear together; praise and please save. That's almost certainly not how the phrase's meaning developed, but it's interesting that it still connects.

It probably shows a character defect, but I don't like roller coasters.

Blue Letter Bible is a nice resource, but be careful teasing meanings out of word roots.