sigma

1888, "lisping difficulty in pronouncing 'S,'" from combining form of sigma + -ism. As "use or recurrence of 'S'" from 1889. Also sigmatismus.

"shaped like the Greek letter sigma" in one of its forms, hence either "shaped like a C" (1660s) or "shaped like an S" (1786). From sigma (q.v.) + -oid. Especially of the flexure of the colon (1891). Related: Sigmoidal; sigmoidally.