or (from stem oro- “up, rise, high”) + thanc (“split, forked”)
Sindarin
orthanc
place name. Mount Fang, Forked Height
Orthanc
noun. forked hight
Noldorin
orthanc
place name. Mount Fang
Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!
Early Noldorin
orthanc
adjective. unbroken, without break or cleavage, united, continuous; masked
A word for “unbroken” and also “without break or cleavage, united, continuous” in Early Noldorin Word-lists of the 1920s (PE13/164-165), a combination of ᴱN. thanc with the privative prefix ᴱN. or-. In one place Tolkien said orthanc meant “unmasked” = ᴱN. {ithanc >>} idanc where the second element appears to be ᴱN. tanc (PE13/156), though the meaning of tanc in this period was “firm” (PE13/153) which seems inappropriate.
Saruman’s fortress, translated “Mount Fang” (LotR/555) or “Forked Height” (UT/400). This name appears to be a combination of a shorter form of orod “mountain” with thanc “cleft, split” (RC/625).
Conceptual Development: In Lord of the Rings drafts from the 1940s, this name was already N. Orthanc (TI/132), but its translation was revised “Stony Heart” >> “Stone Fang” >> “Mount Fang” (WR/35).