sibillant

Originally, the Cyrillic letter fita was borrowed from Greek and corresponded to the Theta, but it's unclear if it ever had a sibillant quality in Slavic; thus correspondence was most likely graphic, not phonological, since the earliest period.

languagehat.com: SCUTCH. 2005

"Stop!" she commanded, coming nearer and dropping her voice to a sibillant whisper.

Idle Hour Stories Eugenia Dunlap Potts

And though the words were slow, deliberate, clean-cut, there was a hissing prolongation of the one sibillant that gave the impression of the 'scape-valve of some pent-up power that bore a ton to the square inch.

Marion's Faith. Charles King 1888