Primitive elvish
reth
root. remain in same place
(g)reth
root. aid, support
res
root. remain in same place
Sindarin
reth
often in names
reth
noun. *climber
reth-
verb. to remain in same place
Quenya
retto
noun. *climber
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
Early Primitive Elvish
reþe
root. aid, support
A root given as ᴱ√RESE “aid, support” (gloss marked with a “?”) in the Qenya Lexicon of the 1910s, but Tolkien later decided that it was confused with another root ᴱ√reðe which seemed to do with kinship (QL/79). After this change, forms derived from ᴱ√RESE “aid” probably still included ᴱQ. resta in the sense “support”, as well as ᴱQ. resta- “aid, come to aid of, rescue, save”. In the contemporaneous Gnomish Lexicon Tolkien has a similar set of words likewise derived from distinct reth- vs. redh- (GL/65), indicating the true form of ᴱ√RESE was probably ᴱ√REÞE, since þ became s in Qenya. As Gnomish derivatives of reth- Tolkien initially gave G. rest, G. resta-, G. reth- (GL/65), but these were deleted leaving only G. grest “succour, salvation”, G. gresta- “aid, help, succour, support”, and G. gretha- “save, rescue, preserve” (GL/42).
It is possible the root ᴱ√REÞE reemerged as √REK “recover, get out/away, save from ruin/peril/loss” given in Tolkien’s later writings as the basis for S. edraith “rescue, saving” (PE17/38).
Neo-Eldarin: For purposes of Neo-Eldarin I think it is worth positing a (Sindarin only) Neo-Root ᴺ√(G)RETH to salvage some of these Gnomish words.
rese
root. aid, support
Gnomish
reth-
verb. to save, rescue, preserve
gretha-
verb. to save, rescue, preserve
English
Rethe
Rethe
The name represents a modernization of the Old English name for March, Hreþmōnaþ, meaning "rough-month".
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.