A word/suffix in 1957 Notes on Names (NN), based on the root √RET and an element in the name S. Orodreth “Mountaineer” (PE17/182). It is thus likely means something like “✱climber”.
Primitive elvish
reth
root. remain in same place
(g)reth
root. aid, support
res
root. remain in same place
A root appearing in a list of Sindarin words placed with Definitive Linguistic Notes (DLN) from 1959, given as √REÞ or √RES “remain in same place, hence be unmoved, patient”. It had the derivatives S. andreth “patience”, which appeared elsewhere as the name of a Bëorian woman (MR/305), and S. Arodreth “Noble Endurance”, a name appearing nowhere else. The variant √RES may have been intended to be the Quenya form.