This root first appeared in The Etymologies of the 1930s as ᴹ√PHIR “die of natural causes” with derivatives like ᴹQ. fire/N. feir “mortal man” and ᴹQ. firin/N. fern “dead” (Ety/PHIR; EtyAC/ÑGUR). In one place it had a rejected variant ᴹ√SPIR (EtyAC/ÑGUR). In later notes, Tolkien explained that √PHIRI meant “exhale, expire, breathe out” and was initially unconnected to death (WJ/387). In this scenario, √PHIRI came to be associated with death through the passing of Q. Míriel, the most notable Elf to die of non-violent causes who “overcome by a great sorrow ... gave up her life in the body and went to the keeping of Mandos, [with] a deep sigh of weariness” (WJ/387). In this event, she was given the new name Q. Fíriel “She that died” but also meaning “She that sighed” (MR/250). From there it came to be used of the natural death of mortal men, something which the Elves had little experience with themselves.
Sindarin
fern
phir
_, the word fern means "dead" with reference to the (inevitable) death of mortals. Also gwann (departed), lenited wann; pl. gwain