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]