sibilancy

from The Collaborative International Dictionary of English v.0.48

Sibilance \Sib"i*lance\, Sibilancy \Sib"i*lan*cy\, n.
   The quality or state of being sibilant; sibilation.
   [1913 Webster]

         Milton would not have avoided them for their sibilancy,
         he who wrote . . . verses that hiss like Medusa's head
         in wrath.                                --Lowell.
   [1913 Webster]