Sometimes “most” is used as an intensifier rather than a superlative:
“Lucy expressed herself most eloquently.”
“The employees work most efficiently.”
There are other degree adverbs that do this, like “quite” and “rather”:
“Tom paints quite beautifully.”
“They got there rather quickly.”
However, to my mind, the latter examples can take on the meaning of both “very” and “somewhat” depending on the context or emphasis, while “most” only takes on the meaning of “very”. “Most” is not working as a superlative or comparing the extent of Lucy’s eloquence to any other type of eloquence or the efficiency of the employees to that of anyone else's employees (as in much/more/most); it is simply adding emphasis. I understand that what may be setting “most” apart from the others could, then, just be the nature of the word's link to forming superlatives, which means we find it difficult accept it with the sense of “somewhat”. My question is whether there is an actual grammatical difference between these forms, or if it really is just its association with forming superlatives that limits its meaning? And if there is a grammatical difference, are there other examples of this?
Also, as far as my personal knowledge of English usage goes, using “most” in this way is a little outdated and sounds upper-class, while the latter two aren’t and don’t (at least definitely not to the same extent). All three can serve the same function, so is there a historical or linguistic reason for why “quite” and “rather” used in this way are still contemporary and “more” isn't?