This word had a quite lengthy history as an element in the name S. Glorfindel “Golden Hair”. It appeared in the Gnomish Lexicon of the 1910s as G. finn “a lock of hair” (GL/35), simply as ᴱN. find or finn “hair” in Early Noldorin Word-lists (PE13/143), and as Old Noldorin sphinde “lock of hair” from The Etymologies of the 1930s under the root ᴹ√SPIN (Ety/SPIN). In notes from the mid-1960s Tolkien said that find, finn meant a “single hair (of man or elf)” vs. S. †findel for a head of hair (PE17/17), but in The Shibboleth of Fëanor from 1968 Tolkien said it meant “tress” and was derived from primitive ✶phindē (PM/362 note #37).
Neo-Sindarin: For purposes of Neo-Sindarin, I’d use fîn for a single hair, find for hair in general or for a tress or lock of hair, and finnel for an entire head of hair.
n. single hair (of man or elf). >> finn