A word for “door” in the name Fen Hollen “Closed Door” (LotR/826; RC/550). In notes from December 1959 (D59), Tolkien based it on the root √PHEN and gave its Quenya equivalent as fenna, indicating a primitive form of ✱phennā (PE17/181). If so, its ordinary form should be fenn, and this was indeed the form in Lord of the Rings drafts from the 1940s (WR/341). Perhaps fen is a reduced pseudo-prefixal form.
Conceptual Development: In The Etymologies of the 1930s Tolkien had N. fenn “threshold” derived from ON. phenda under the root ᴹ√PHEN (Ety/PHEN).
Neo-Sindarin: I don’t think the senses “door” and “threshold” are likely to coexist, so for purposes of Neo-Sindarin I would limit fenn to “door” and would use ᴺS. fend < ✱phenda for “threshold”, following the principle that nd remained “at the end of fully accented monosyllables” in Sindarin (LotR/1115).
_ n. _door. Q. fenna. >> fennas