A word appearing as fend “door” in notes on the Common Eldarin Article (CEA) from 1969 (PE23/136). In notes from December 1959 (D59), Tolkien gave it as fen “door” derived from the root √PHEN, with a Quenya equivalent as fenna indicating a primitive form of ✱phennā (PE17/181). In The Lord of the Rings proper, it was an element in the name Fen Hollen “Closed Door” (LotR/826; RC/550); perhaps fen is a reduced pseudo-prefixal form of fenn/fend.
Conceptual Development: In The Etymologies of the 1930s Tolkien had N. fenn “threshold” derived from ON. phenda under the root ᴹ√PHEN (Ety/PHEN). In Lord of the Rings drafts from the 1940s it appeared as fenn in Fenn Forn(en) and similar variants, all earlier names for Fen Hollen (WR/341).
Neo-Sindarin: I don’t think the senses “door” and “threshold” are likely to coexist, and for purposes of Neo-Sindarin I would limit fend to “door”.
_ n. _door. Q. fenna. >> fennas