Source : Webster's Revised Unabridged Dictionary (1913)

Sigmoid \Sig"moid\, Sigmoidal \Sig*moid"al\, a. [Gr. ???; ???
   sigma + ??? form, likeness: cf. F. sigmo["i]de.]
   Curved in two directions, like the letter S, or the Greek
   [sigmat].

   {Sigmoid flexure} (Anat.), the last curve of the colon before
      it terminates in the rectum. See Illust. under
      {Digestive}.

   {Sigmoid valves}. (Anat.) See {Semilunar valves}, under
      {Semilunar}.