In his Unfinished index for The Lord of the Rings, Tolkien translated the Sindarin name Narchost as "bitter-biting fort". Its initial element narch "bitter-biting" is not otherwise attested, but a similar (Noldorin) verb narcha- "to rend" appears in the Etymologies as a derivative of the root NÁRAK. The second element is ost "fortress".
Sindarin
narchost
place name. Bitter-biting Fort
Narchost
Narchost
One of the Towers of the Teeth along with its companion Carchost (LotR/900), translated “Bitter-biting Fort” in Tolkien’s “Unfinished Index” of The Lord of the Rings (RC/601). This name is a combination of narch “fang” and ost “fort(ress)”.
Conceptual Development: When it was first named specifically in Lord of the Rings drafts from the 1940s, this fort was already called N. Narchost (SD/23). The earlier name N. Nargos may have been a precursor to this name (TI/344), although that name referred to a fort near Cirith Ungol rather than the Gates of Mordor.