Well, while the name of the location she had her lair in is in Sindarin (”Spider’s Cleft”) the name she’s known by seems to be not Sindarin.
Basically “Shelob” seems to be derived from Old English, Germanic roots, and rather is not her real name, but a representation of a Westron name (pretty much like “Frodo” isn’t Maura’s actual name, but an adaptation), as it seems to be a combination of “she” + “lob” as derived from Old English “lobbe/loppe” meaning “spider” as from Proto-Germanic *lubbō, *lubbǭ (“that which hangs or dangles”) thus she’s simply a female spider
In Sindarin her name could possibly be something like “Ungoleth” probably (”ungol” - “spider” + feminine ending “-eth”)