assibilate
Concise Oxford English Dictionary © 2008 Oxford University Press:
assibilate /əˈsɪbɪleɪt/
▶verb Phonetics pronounce (a sound) as a sibilant or affricate ending in a sibilant (e.g. sound t as ts).
– derivatives
assibilation noun.
assibilation noun.
– origin C19: from L. assibilat-, assibilare, from ad- ‘to’ + sibilare ‘to hiss’.
'assibilate' also found in these Oxford entries:

