According to the Wikipedia page on Orcus, the name for Orcs seems to have come from Orcus. I'm sure someone else out there in bloglandia already made this discovery, but it's news to me!
Here are the relevant passages (as of today, anyway) from the Orcus article:
From Orcus' association with death and the underworld, his name came to be used for demons and other underworld monsters, particularly in Italian where orco refers to a kind of monster found in fairy-tales that feeds on human flesh. The French word ogre (appearing first in Charles Perrault's fairy-tales) may have come from variant forms of this word, orgo or ogro; in any case, the French ogre and the Italian orco are exactly the same sort of creature. An early example of an orco appears in Ludovico Ariosto's Orlando Furioso, as a bestial, blind, tusk-faced monster inspired by the Cyclops of the Odyssey; this orco should not be confused with the orca, a sea-monster also appearing in Ariosto.
This orco was the inspiration to J. R. R. Tolkien's orcs in his The Lord of the Rings. In a text published in The War of the Jewels, Tolkien stated:
Note. The word used in translation of Q[uenya] urko, S[indarin] orch, is Orc. But that is because of the similarity of the ancient English word orc, 'evil spirit or bogey', to the Elvish words. There is possibly no connexion between them. The English word is now generally supposed to be derived from Latin Orcus.Also, in an unpublished letter sent to Gene Wolfe, Tolkien also made this comment:
Orc I derived from Anglo-Saxon, a word meaning demon, usually supposed to be derived from the Latin Orcus—Hell. But I doubt this, though the matter is too involved to set out here.