"Nazgul" and "ghoul"
- Tolkien was fond of linguistic jokes, of course, and his selection of
the word "Nazgul" might be one of them.
http://www.dictionary.com/cgi-bin/dict.pl?term=ghoul tells us that a
ghoul was (definition 3) "an evil spirit or demon in Muslim folklore
believed to plunder graves and feed on corpses". When Tolkien was
still writing the earlier drafts of LOTR, the Ringwraiths were
related to the Barrow-wights. Tolkien often picked names because he
liked the sound of them, and then worked them into his languages.
Perhaps he did something similar with "Nazgul" and "ghoul", even
though the Black Speech compound is "nazg" + "ul".
"[Arabic gul, from gala, to seize, snatch. See gwl in Semitic Roots.]"