Source : Webster's Revised Unabridged Dictionary (1913)

Sibilance \Sib"i*lance\, Sibilancy \Sib"i*lan*cy\, n.
   The quality or state of being sibilant; sibilation.

         Milton would not have avoided them for their sibilancy,
         he who wrote . . . verses that hiss like Medusa's head
         in wrath.                                --Lowell.