Joe Wright, who had played a key role in setting the young Tolkien on the philological path that would occupy him to the end of his days, produced a stunning six-volume work of scholarship, The English Dialect Dictionary, from 1898 to 1905, in which Volume I contains a substantial entry for attercop (91).
Returning to real etymology, the first element in attercop goes back to Old English ator (variously spelled, ater, attor, etor, etc.
We have thus found a kind of double meaning--"head" versus "cup"--is the second element of attercop.
9) As it happens, there is a Finnish word, myrkky, which is quite close phonologically, but which doesn't mean "dark" at all; rather, it means "poison," just like the first element of attercop.
Tolkien used the word attercops himself in early drafts of the poem "Errantry," published in The Oxford Magazine in 1933 and probably composed a few years before.
Tolkien's Attercops live in a forest called Mirkwood.