Definify.com
Webster 1913 Edition
Sibilate
Sib′i-late
,Verb.
 T.
 & I.
 To pronounce with a hissing sound, like that of the letter s; to mark with a character indicating such pronunciation. 
Definition 2025
sibilate
sibilate
English
Verb
sibilate (third-person singular simple present sibilates, present participle sibilating, simple past and past participle sibilated)