sibilate in WordNet English dictionary

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