Is it ensure, insure, or assure?
1660s, from Latin sibilantem (nominative sibilans), present participle of sibilare "to hiss, whistle," possibly of imitative origin (cf. Greek sizein "to hiss," Lettish sikt "to hiss," Old Church Slavonic svistati "to hiss, whistle"). Related: Sibilance; sibilation (1620s).
"speech sound having a hissing effect," 1772, from sibilant (adj.).
sibilant sib·i·lant (sĭb'ə-lənt)
Of, characterized by, or producing a hissing sound like that of (s) or (sh).
in phonetics, a fricative consonant sound, in which the tip, or blade, of the tongue is brought near the roof of the mouth and air is pushed past the tongue to make a hissing sound. In English s, z, sh, and zh (the sound of the s in "pleasure") are sibilants. Sometimes the affricates ch and j are also considered as sibilants. See also fricative.