Dan. urc n. “goblin”
A noun for “goblin” developed from primitive ᴹ✶orku, also attested in its plural form yrc (Ety/ÓROK). The change of [o] to [u] is hard to explain, as noted by Helge Fauskanger (AL-Nandorin/urc). In an earlier version of the entry Tolkien wrote Dan. orc (EtyAC/ÓROK), which is the expected form. The simplest explanation is that this form actually developed from a variant primitive form *✶urku. There is a similar issue with Ilk. urch, so perhaps this variant was used by those Eldar who did not complete the journey to Valinor.
References ✧ Ety/ÓROK; EtyAC/ÓROK
|ᴹ✶órku > urc||[orku] ? [urku] > [urk]||✧ Ety/ÓROK|