In his Notes on Nomenclature, Tolkien mentions, that he had been inspired by the orc-nēas - there used to describe monsters ("un-death", "demon-corpses") - and and the orc-ţyrs ("orc-giants") in Beowulf. The name Orcs - like orc-nēas and orc-ţyrs - is probably derived from Latin orcus ('underworld', 'realm of the dead') and - like the Greek hades - the name denotes also the god of this 'underworld'.
The name also strongly reminds of ogres, those mysterious sea-animals, often portrayed as 'sea-giants' or 'sea-monsters', which are mentioned briefly in Tolkien's translation of Sir Gawain and the Green Knight, though Tolkien points out, that he sees no connection of his Orcs with those ogres.
It appears likely, that Tolkien had different "inspirations" or "sources" for his Orcs, on the one hand the orc-nēas in Beowulf on the other hand, especially when considering the connection between Elves and Orcs in parts of the Silmarillion, they remind strongly of the swartâlfar "black elves" or döckâlfar "dark elves", who appear in the Germanic mythology.
The swartâlfar there seem to be more closely related to - or at times identical with - dwarves lateron, as can be gathered from Jakob Grimm in his Deutsche Mythologie (c.f TolkiensSources), or from Sturluson's Edda:
|Then Allfather sent him who is called Skírnir, Freyr's messenger, down into the region of the Black Elves, to certain dwarves, and caused to be made the fetter named Gleipnir.|