Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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