ᴹ√SUS root. “hiss”
A root in The Etymologies of the 1930s glossed “hiss” with the derivative ᴹQ. surya “spirant consonant” (Ety/SUS). The root ᴹ√SIS was written nearby and was likely intended to be a variant as suggested by Carl Hostetter and Patrick Wynne (EtyAC/SIS). These two variants might be later iterations of primitive ᴱ✶sṣt- from the Early Noldorin Dictionary of the 1920s with derivatives like ᴱQ. histe- “hiss” (PE13/163).
References ✧ Ety/SUS; EtyAC/SIS