Rather than from the Old English word for "boar", eofor (German Eber) Tolkien derives the word from eafor "packhorse", from a verb aferian "transport", related to Middle English aver "draught-horse", a word surviving in northern dialects. The Proto-Germanic root *ab- "energy, vigour, labour" of the word is cognate to Latin opus.
J. R. R. Tolkien, The devil's coach-horses in: The Review of English Studies: a quarterly journal of English literature and the English language; vol. 1, no. 3 (July 1925), pp. 331–36 London: Sidgwick & Jackson , reprinted 1969