Did the opaque letter names at the end of the Slavic alphabets, fert, kher, tsy and sha among them, ever carry a sense in Slavonic?

The first twenty letter names of both Slavic alphabets are ordinary Slavonic words and read together as connected sentences, from azu buky vedi, I know letters, through rtsy slovo tverdo, say the word firmly. After that point the names stop meaning anything recoverable. The reference literature records them as of uncertain or unknown origin and offers no reading. This is not a disagreement between scholars with rival etymologies; it is an absence. Nobody has established whether these names are corrupted Slavonic words, reflexes of Greek letter names, or labels that never meant anything at all, and the question has not been taken up.