§ Hebel Dúath >> Ephel Dúath (WR:122)
These are Elvish names for the ’Mountains of Shadow’, a mountain range marking the southwest of Mordor, translated as ’Fence of shadow’ in the unfinished index (RC:457) with ephel ’outer fence’ < et-pel < pel- ’go round, encircle’, compare Et-pele > Eppele > Ephel (WR:137), ephel < eppel < et-pel ’outer wall or fence’ (PE17:65), PEL- ’revolve on fixed point’ in The Etymologies; and dú(w)ath ’night-shade’ (DOƷ-, DÔ-).
If the earlier hebel is derived from PEL- as well, it needs a prefix *he- instead of et-, but nothing like this is attested. Instead we may assume that the underlying root is √khep- ’retain, keep, do not give away or release, keep hold of’ (VT41:6, PE17:157) – as also in the Sindarin verb ú-chebin lit. *’I do not keep’ (LotR App.A) – with the meaning ’fence’ derived from it. For the formation compare keglē > S. cail ’a fence or palisade of spikes and sharp stakes’, thus perhaps khepelē > hebel with ómataima.