I've often wondered about these words in the Tantum ergo. Honour we can give to God, as is his due; Might we cannot give Him, because he possessses it, but we can and should doxologically ascribe it to him, acknowledging that it is his. But "Salus", Salvation, seems to me a different Kettle of Fish: it is in principle what he bestows upon us. In what sense can we 'give' it to him, or say "let it be" to him?
I wondered if Latin philology might help; given the root meaning, does it here mean 'perfection', which we could ascribe to God? Or, in view of the phrase "dat salutem", "gives greeting", is that the sense here? But it seems unlikely that S Thomas is delving into antiquated Indo-European philology; or that the phrase is simply a way of saying "Hello, God".
I suspect S Thomas got the phrase from the old hymn to S Martin, Iste Confessor, eighth century and probably Carolingian (they liked Sapphics), where the doxology begins "Sit salus illi, decus atque virtus ...". But there is a Biblical basis: Revelation 7:10 "Salus Deo nostro" (the Greek is "He Soteria toi theoi hemon" ... see also 12:10 and 19:1). R H Charles (still my favourite commentary on Revelation) comments that "They know and proclaim that the Deliverance is not their own achievement, but that of God and of the Lamb".
So are we really saying, in these doxologies, "We ascribe our Salvation to God's action"?.
I would be glad if anyone has spotted something textual, literary, or historical that I have missed.
By the way, Iste Confessor used to be the Office Hymn for all 'Confessors' (i.e. male Saints who were not martyrs) in the Old Rites. Dom Lentini's coetus commented "The very few metrical licenses led the Urbanian correctors to make so many and such grave changes that they gave pretty well a new appearance to the hymn. It ought to be totally restored; it is very well known and worthy and not to be restricted simply to the feast of S Martin".
But when Liturgia Horarum came out, Lentini had been overruled, the hymn confined to S Martin, and some very unmemorable compositions had been provided for every category of male non-martyr.