What is "assibilate"

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.

vb. 1 (context linguistics phonetics English) To change into or pronounce with the accompaniment of a sibilant sound or sounds. 2 (context linguistics phonetics English) To change by assibilation.

1. v. insert a sibilant sound before or after (another sound)

2. change into a sibilant; "In the syllable /si/, the /s/ sibilates in Japanese"