The name Finduilas is not explained. Fin- is surely 'hair' in this context, but -duilas is less clear. -las as a name element is typically 'leaf' (as in Legolas) but that does not seem to fit as part of the interpretation of Finduilas' name. The most likely interpretation would be a connection to the root dui-, 'flow', so Finduilas' name may be a reference to her flowing hair.
A more elaborate alternative would be fin-duil-as, roughly 'hair like sun-gleam on a river'. This is a rather awkward construction to say the least, but it does help to explain Gwindor's choice of the name Faelivrin ('gleam of the Sun on the Pools of Ivrin'), which becomes a play on the meaning of Finduilas.