Definition of "sibilant"

She had a curious habit of prefacing everything she said with a soft sibilant sound."S-s-s Grace," she said, "it's just like I was telling Brother Hutson the other day. 'S-s-s Brother Hutson,' I said, 'looks like we're fighting a losing battle, a losing battle.' I said."

1960 July 11, Harper Lee, To Kill a Mockingbird, Philadelphia, Pa., New York, N.Y.: J[oshua] B[allinger] Lippincott Company

Groove fricatives all have more or less of an [s]-like quality, and are for this reason sometimes called sibilants.

1955, H. A. Gleason, An Introduction to Descriptive Linguistics, page 194, section 14.7