Early Primitive Elvish
sisi
root. singe, etc.
þisi Reconstructed
root. singe, etc.
There are more words but they are from Tolkien's earlier conceptional periods. Tolkien likely changed these words as he evolved the aesthetics and completeness of the languages. You may even find languages that Tolkien later rejected. Do not mix words from different time periods unless you are familiar with the phonetic developments.
Below: Tolkien's Late Period (1950–1973). These are from Tolkien's late conceptual period — his most settled forms of the languages. Prefer these over the earlier ones above when composing new text.
The root ᴱ√SISI “singe, etc.” appeared in the Qenya Lexicon of the 1910s with derivatives ᴱQ. sisi- “scorch, singe, fry” and ᴱQ. sisin “parched, scorched” (QL/84). Its derivatives in the contemporaneous Gnomish Lexicon were words like G. thisin “parched, withered” and G. thith “dust” (GL/73), indicating an actual root form ✱ᴱ√ÞISI.
Neo-Eldarin: For purposes of Neo-Eldarin I think it is worth positing a Neo-Root ᴺ√THITH to salvage some of these early words; ᴺ√THITH is better than ✱ᴺ√THIS if we wish retain Early Qenya forms, as otherwise they would become sir- rather than sis-.