Source : Webster's Revised Unabridged Dictionary (1913)

Assibilate \As*sib"i*late\, v. t. [L. assibilatus, p. p. of
   assibilare to hiss out; ad + sibilare to hiss.]
   To make sibilant; to change to a sibilant. --J. Peile.

Source : WordNet®

assibilate
     v 1: insert a sibilant sound before or after (another sound)
     2: change into a sibilant; "In the syllable /si/, the /s/
        sibilates in Japanese"