The Collaborative International Dictionary of English v.0.48:

Assibilation \As*sib`i*la"tion\, n.
   Change of a non-sibilant letter to a sibilant, as of -tion to
   -shun, duke to ditch.
   [1913 Webster]

WordNet (r) 3.0 (2006):

assibilation
    n 1: the development of a consonant phoneme into a sibilant
    2: pronunciation with a sibilant (hissing or whistling) sound
       [syn: sibilation, assibilation]