A preposition meaning “through” in The Etymologies of the 1930s derived from ON. trī under the root ᴹ√TER(ES) “pierce” (Ety/TER), so presumably derived from ✱trē with ancient ē > ī as usual in Noldorin and Sindarin. In The Etymologies as published in The Lost Road Christopher Tolkien gave the Noldorin and Old Noldorin forms as trî and trí following the usual orthographic conventions of those languages (LR/392), but Carl Hostetter and Patrick Wynne indicated they were both trī in their Addenda and Corrigenda to the Etymologies (VT46/18).
Noldorin
godrebh
adverb. abreast; *together-through
godrebh
adverb. through together
trî
preposition. through
tre-
prefix. through (but denoting completeness when prefixed to verbs, cf. English idioms like "talk something through")
tre-
prefix. through
A prefixal form of N. trî “through” in The Etymologies of the 1930s, appearing as tre- when unstressed (the norm) and tri- when stressed, though there are no examples of the latter (Ety/TER). Presumably this refers to ancient rather than modern stress.
tri
prefix. through (but denoting completeness when prefixed to verbs, cf. English idioms like "talk something through")
trî
preposition. through
The word N. godrebh appeared in a phrase on a draft of Thrór’s Map written in 1936 (TAI/92). The English translation of the phrase implies the meaning of the word is “abreast” but the corresponding Old English translation has “samod þurgh”, which in modern English means “together through”. This make it likely that this word is a combination of N. go- “go” and a variant of N. trî “through” < trē (Ety/TER, WŌ). It is probably derived from primitive ✱wotrĕb(e), where the ancient suffix -be is probably adverbial.
Neo-Sindarin: Most Neo-Sindarin writers update this form to ᴺS. godref to be more consistent with normal Sindarin spellings, as suggested in HSD (HSD).