Ancient Greek did not have a /ʃ/ phoneme, so the derived Greek letter sigma (Σ) came to represent the voiceless alveolar sibilant /s/.
While the letter shape Σ continues Phoenician šîn, its name sigma is taken from the letter samekh, while the shape and position of samekh but name of šîn is continued in the xi. 
Within Greek, the name of sigma was influenced by its association with the Greek word σίζω (earlier *sigj-) “to hiss”. The original name of the letter “sigma” may have been san, but due to the complicated early history of the Greek epichoric alphabets, “san” came to be identified as a separate letter, Ϻ. Herodotus reports that “San” was the name given by the Dorians to the same letter called “Sigma” by the Ionians.
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