sibilant vs sibilous

sibilant

adj

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

noun

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

sibilous

adj

  • Having a hissing sound; sibilant. 

How often have the words sibilant and sibilous occurred in a corpus of books? (source: Google Ngram Viewer )