C comes from the same letter as G or g. The Semites named it gimel. The sign is possibly adapted from an Egyptian hieroglyph for a staff sling, which may have been the meaning of the name gimel. Another possibility is that it depicted a camel, the Semitic name for which was gamal.
In the Etruscan language, plosive consonants had no contrastive voicing, so the Greek Γ (Gamma) was adopted into the Etruscan alphabet to represent the /k/ phoneme. Already in the Western Greek alphabet, Gamma first took a form in Early Etruscan, then in Classical Etruscan. In Early Latin it took a form then C in Classical Latin. Early Latin used C for both /k/ and /g/, but during the 3rd century BC, a modified character was introduced for /g/, and C itself retained for /k/. Hence, in the classical period and after, G was treated as the phonetic representative of "gamma", and C as the equivalent of "kappa", in the transliteration of Greek words into Roman spelling, as in "KA∆MOΣ, KYPOΣ, ΦΩKIΣ," in Roman letters "CADMVS, CYRVS, PHOCIS". It is also possible but uncertain that C represented only /g/ at a very early time, while K might have been used for /k/.
Other alphabets have letters identical to C in form but not in use and derivation, in particular the Cyrillic letter Es which derives from one form of the Greek letter sigma, known as the "lunate sigma" due to its resemblance to the crescent moon.