sibilant

adj

1. Characterized by a hissing or hushing sound such as the s or sh in sack or shack.

   “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."”

noun

1. A consonant having a hissing or hushing sound such as the s or sh in sack or shack.

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