The unglossed root ᴱ√SṢT͡YṢ appeared in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. sastya “sore, galled” and ᴱQ. sist (sisty-) “ulcer, sore” (QL/86). The latter word was also mentioned as siste in “ulcer, boil” as a derivative of ᴱ✶sṣtē (PE12/14), and the primitive form was given as sistyi, though Tolkien did say it was indicative of ancient syllabic ṣ (PE12/3). However, in the Early Noldorin Dictionary of the 1920s it seems primitive ᴱ✶sṣt- = “✱hiss” (PE13/163); see the entry on ᴹ√SUS “hiss” for discussion.
Neo-Eldarin: For purposes of Neo-Eldarin I think it is worth positing a Neo-Root ᴺ√SISTI to salvage some of these early words.