The Collaborative International Dictionary of English v.0.48:

Sigmoid \Sig"moid\, Sigmoidal \Sig*moid"al\, a. [Gr. ???; ???
   sigma + e'i^dos form, likeness: cf. F. sigmo["i]de.]
   Curved in two directions, like the letter S, or the Greek
   [sigmat].
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sigmoid
    adj 1: curved in two directions (like the letter S)
    2: of or relating to the sigmoid flexure in the large intestine
       [syn: sigmoid, sigmoidal]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "sigmoid":
   Cynthian, S-shaped, ambagious, anfractuous, bicorn, circuitous,
   circumlocutory, convolutional, corniform, crescent,
   crescent-shaped, crescentic, crescentiform, crescentlike, falcate,
   falciform, flexuose, flexuous, horn-shaped, horned, involute,
   involuted, involutional, labyrinthine, lunar, lunate, luniform,
   lunular, mazy, meandering, meandrous, menisciform, meniscoid,
   moon-shaped, moonlike, rivose, rivulose, roundabout, ruffled,
   semicircular, semilunar, serpentine, sickle-like, sickle-shaped,
   sinuate, sinuose, sinuous, snaky, torsional, tortile, tortuous,
   turning, twisting, twisty, two-horned, whorled, winding,
   wreathlike, wreathy