Original: This letter "C" stood for "coffin" or "Container", the rocket being stored in a semi-hardened container; it was prepared for launch by being raised and fueled in the open.
Simplified: The missile was originally given the military designation "XB-65", thus making it a bomber; from 1955 it was redesignated "SM-65" and, from 1962, it became "CGM-16". This letter "C" stood for "coffin" or "Container", because the rocket was stored in a semi-hardened container; it was prepared for launch by being raised and fueled in the open.