The story
Developed by J. R. R. Tolkien as a language in his fantasy works, in which the word Sindarin itself is not of Sindarin origin: it is a term in Quenya (another of Tolkien's constructed languages), derived from Sindar, the Quenya name for the people who spoke it, which in turn is derived from sinda (“grey”).
Etymology from Wiktionary, CC BY-SA 4.0.