transitive verb. To pronounce with a hissing sound; make sibilant.

To render sibilant, as a sound; change into a sibilant or hissing sound; alter, as a sound, by the phonetic process called assibilation.

transitive verb. To make sibilant; to change to a sibilant.

verb. To change into or pronounce with the accompaniment of a sibilant sound or sounds.

verb. To change by assibilation.

verb. insert a sibilant sound before or after (another sound)

verb. change into a sibilant

"(Lat. e, oe, ae) and i the c had already begun to assibilate in Latin itself; in O.Sp. it yielded the voiceless dental sibilant c"

change  sibilate  

assibilated  assibilating