The Collaborative International Dictionary of English v.0.48:

Sibilate \Sib"i*late\, v. t. & i.
   To pronounce with a hissing sound, like that of the letter s;
   to mark with a character indicating such pronunciation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sibilate
    v 1: pronounce with an initial sibilant
    2: utter a sibilant
    3: express or utter with a hiss [syn: hiss, sizz, siss,
       sibilate]
    4: make a sharp hissing sound, as if to show disapproval [syn:
       hiss, siss, sizz, sibilate]