What Good Is a Dictionary?

I was more than a little shocked today researching the term "moderato" in scholarly works about Haydn at the extent to which the word is accepted as meaning something like "medium."  Come on...  really?  Medium what?

I want to know who used the term first to mean what I think it means, which is lyrical rather than metric.  What a later era might have called "tempo rubato." 

It seems fairly clear to me that written music in the 18th century had to conform pretty much to standard practice; if there were to be deviations in terms of timing they had somehow to be hinted at but not made so explicit as to turn everybody off.

The trouble is that we are still being taught that the standard practice corresponds to real content when I think that has to be questioned as often as not.