Sigma

google

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Greek.

wiktionary

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From Ancient Greek σῖγμα(sîgma), σίγμα(sígma), probably from the root sig- as seen in σίζω(sízō) (< sig-yo "to hiss") + -μα(-ma). [1] [2]

etymonline

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sigma

18th letter of the Greek alphabet, corresponding to Latin S, a metathesis of Hebrew samekh. In uncial writing, shaped like an S or a C.