The name Glorfindel through time [external time]
Name List to the Fall of Gondolin (NFG): Glorfindel 'Gold-tress' Gnomish Lexicon (GL): glôr 'gold' findel 'lock of hair' [findel was struck through however] fith (fidhin) 'a single hair' Later entries: finn 'lock of hair' and fingle or finnil 'tress' In another place in GL: Glorfindel 'Gold-locks' was changed to Glorfinn with a variant Glorfingl
Qenya Lexicon (QL): 'laure [much the same as 'kulu'] laurina 'golden' laure is the final l of tilkal (p. 100 where it is said to be the 'magic' name of gold, as ilsa of silver). The Gnomish words are glor 'gold' glorin, gloriol 'golden' but GL gives no names of the Golden Tree' [citation from The Book of Lost Tales I, Appendix].
Interestingly, according to NFG: 'Findel is 'tress' and is the Elfin Findil' and in the Qenya Lexicon [early Quenya] we find: findl 'lock of hair' and firin 'ray of sun'.
It might be noted that at this stage, Tolkien sometimes used 'Elfin' in distinction to Gnomish, when referencing languages. Jumping to a document called Etymologies, dated roughly to the mid to later 1930s (emphasis on roughly). Here we are dealing with the language Noldorin [not yet Sindarin], thus 'N' in the citations, which existed before a major revision in the internal history of the languages was to take place.
Etymologies root GLAW(-R-): Q laure gold (properly the light of the Tree Laurelin) N glaur 'gold' [reduced in polysyllables to glor, lor] Glorfindel, Glaurfindel Etymologies root SPIN-: *spinde 'tress, braid of hair' Q finde, ON sphinde lock of hair, spindele (braided hair); N findel, finnel, cf Glorfindel.
Jumping ahead to ideas written after The Lord of the Rings was published (thus after 1955): now we are finally dealing with Quenya and Sindarin, and although Tolkien would continue to tinker with his languages, the general history was settled on, as described in The Silmarillion and The Lord of the Rings.
Words, Phrases and Passages in The Lord of the Rings (WPP):
S Glorfindel = glaur + phin-dela DEL-, thick, dense Q phindele mass of long hair OS findel, later to finnel The names of the Noldor in legend often contain archaic elements notably retention of nd medially and finally. JRRT, WPP
This is interesting, as Tolkien seemingly explains why we do not have *Glorfinnel (compare to the 'Noldorin' name of Sam's daughter, Glorfinniel).
But it appears JRRT was soon enough dissatisfied with part of the derivation above. He next writes:
'The proposed DEL- 'thick, dense' will not, however do: it does not provide suitable Quenya or Sindarin forms outside the proposed *spin-dele 'mass of hairs, head of hair'. This is best explained from SPIN- 'fine thread, filament' distinct (though probably ultimately connected) from PHIN- 'skilful, neat, clever (especially applied to hands and fingers).' JRRT, WPP
Tolkien will also refer here to the name Finwe 'man of manual skill'. So JRRT then imagines S esbin 'thin thread' find, finn 'single hair' (of man or elf)' *spinde poetic or archaic findel *spindila 'head of hair' (fax), preserved mainly in such old names as Glorfindel.'
But this does not seem to have fully landed either (assuming I have the correct external chronology here), as when Tolkien gets to musing about the name Finrod (which appears later in the books, so that's why I assume the following comments followed the earlier comments above)...
The element Fin- in Nold[orin] names is from PHI/PHINI of which the basic sense seems to have been 'skill, dexterity' (...) Its forms especially in names were however associated with products of SPIN-ID, lock, tress of human/Elvish hair. Q. finde, findele 'tress, lock'. So no doubt in Findis. Cf. Laurefin(de), Laurefindele, Sindarized as Glorfindel. But Findel (so, not to finnel) is prob[ably] an [?adj.] spin-dela, 'having beautiful hair', cf. esbin, a tress. Findel = Fairfax. JRRT, WPP
And in seeming conjunction with this, WPP also reveals a root or base DEL-
'similar to PHIN but passive delya > lelya, fine, beautiful S. deil. spindela, having fine hair. spindidele > S findel. Findel.' And as for the initial element: Q laurie laure, poetic or archaic gold (poetic: not the metal, but rather 'golden light,' properly or mythologically the light of Laurelin, one of the two trees in Valinor. S. glawar as in Glewellin ...' LAW-, LAWAR- (...) the application to gold of this stem was poetic and referred to colour primarily (as especially of laburnum) not to material (malta) S glaw- sunshine Q loar, lavar (golden) blossom, laure (e), golden colour of sunshine or golden flowers. S glawar, glaur- JRRT, WPP
So we arrive at Glorfindel 'Gold (golden light) + hair (fair hair)'.
Is this the final meaning of this name? I can't think of anything later at the moment, and in Last Writings Tolkien mentions that the name was intended to mean 'gold-tressed' back in the old version of the tale. Although JRRT had more to say in The Shibboleth of Feanor, regarding the name Finwe and words for hair.