I thought I had made a grievous error.  In scanning through the Os to find words to eliminate, I found “orcs” but not “orc”.  Baffled that “orc” would be in The Ten Thousand, I checked that list and found “force” and “divorce” but not “orc” by itself.  Frantic that I had mistaken “force” for “orc” at some point and eliminated them all, I raced to my backup copies of words and found the same thing.  The text confirms.  “Orcs” is used twice in the entire text and “orc” none at all, except in the compound word “Orcrist”.  The monsters in the mountains of this work are “goblins”.

Orcs (the word is as far as I am concerned actually derived from Old English orc ‘demon’, but only because of its phonetic suitability) are nowhere clearly stated to be of any particular origin. But since they are servants of the Dark Power, and later of Sauron, neither of whom could, or would, produce living things, they must be ‘corruptions’. They are not based on direct experience of mine; but owe, I suppose, a good deal to the goblin tradition (goblin is used as a translation in The Hobbit, where orc only occurs once, I think), especially as it appears in George MacDonald, except for the soft feet which I never believed in. The name has the form orch (pl. yrch) in Sindarin and uruk in the Black Speech.

The OED says that “orc” is more likely derived from ogre and cites a phrase in Beowulf –  “elves and orcs”.  OED credits Tolkien with reviving an obsolete word.  Because of Tolkien, the word is neither obsolete nor archaic.  Since he revived it, however, I am giving this word those tags.

05.133 the orcs of the mountains
07.151 and orcs of the worst description.

“orc, n.2.” OED Online. Oxford University Press, March 2015. Web. 9 May 2015.

Tolkien, J.R.R. (2014-02-21). The Letters of J.R.R. Tolkien (Kindle Locations 3759-3765). Houghton Mifflin Harcourt. Kindle Edition.