Encountering Sibilant in the novel I am reading, I also discovered it's meaning. Here is the quote - from Where Serpents Sleep, by C.S. Harris.
"A woman with the limp tails of codfish dangling from her apron brushed past with a sibilant hiss, her elbows clearing a path at her waist."
Now that is a sentence that paints a word picture! And here is the definition of Sibilant...
Should you be so inclined to try to verify this definition by making a sibilant hiss of your own, be forewarned, the dog won't enjoy.