Elsewhere in this blog I reported on the first quotation from the OED for the term “flat adverb”:
1871 J. Earle Philol. Eng. Tongue vii. 361 The Flat Adverb is simply a substantive or an adjective placed in an adverbial position.
(This link, as well as the one below, actually works if you are on a university network for instance and if your library subscribes to the OED Online edition.)
John Earle (1824-1903), the author of The Philology of the English Tongue (1871), has an entry in the Oxford Dictionary of National Biography. Earle’s book can be found through Google Books, and it has a fairly lengthy section on the flat adverb, called “Of the Flat Adverb” (1871:361-365). The section suggests that it was indeed Earle who coined the term himself:
The use of the unaltered adjective as an adverb has a peculiar effect, which I know not how to describe better than by the epithet Flat (1871:361).
He considers the use of flat adverbs “rustic and poetic … because it is archaic”, and he adds that it is “all but universal with the illiterate” (1871:364). He may not have used flat adverbs himself, which seems to account for the labels “rustic” and “illiterate”, but it is peculiar that he calls it “archaic”: the flat adverb has never gone out of use.
What is also worth commenting on is that Earle’s book was published by OUP (Clarendon Press): OUP published Fowler’s Modern English Usage as well as many other usage manuals, and we can now see that this interest in books on usage with OUP goes back a long time.