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"shaped like a C" (1660s) or "shaped like an S" (1786), from sigma (q.v.) + -oid. Especially of the flexure of the colon (1891).
sigmoid sig·moid (sĭg'moid') or sig·moi·dal (sĭg-moid'l)adj.
Having the shape of the letter S.
Of or relating to the sigmoid flexure of the colon.