sibilancy

noun

1. (uncountable, usually)The property or state of being sibilant.

   “How do such words differ from hilltop, townend, candlelight, rushlight, cityman, and the like, where no double s can be made the scapegoat? Certainly Milton would not have avoided them for their sibilancy, he who wrote […] verses that hiss like Medusa's head in wrath.”