Voici ce que dit Tolkien à l'entrée Orc page 171 :
Orc : This is supposed to be the Common Speech name of these creatures at that time; it should therefore according to the system be translated into English, or the language of translation. It was translated "goblin" in The Hobbit, except in one place ; but this word, and other words of similar sense in other European languages (as far as I know), are not really suitable. The orc in The Lord of the Rings and The Silmarillion, though of course partly made out of traditional features, is not really comparable in supposed origin, functions, and relation to the Elves. In any case orc seemed to me, and seems, in sound a good name for these creatures. It should be retained.
It should be spelt ork (so the Dutch Translation) in a Germanic language, but I had used to spelling orc in so many places that I have hesitated change it in the English text, though the adjective is necessarily spelt orkish. The Grey-elven form is orch, plural yrch.
I originally took the word from Old English orc (Beowulf 112 orc-nass and the gloss orc = pyrs ("orgre"), heldeofol ("hell-devil"). This is supposed not to be connected with modern English orc, ork a name applied to various seabeasts of the dolphin order.
Voilà donc un texte qui nous éclaire sur l'origine du nom de ces créatures qui nous inspirent tant de question...
Cédric