A word for a “cleft, cut, slash” (PE21/81; Ety/KIRIS) derived from √KIRIS, a blend of the roots √KIR and √RIS (PE17/87).
Conceptual Development: This word dates all the way back to G. criss “cleft, gash, gully” in the Gnomish Lexicon of the 1910s where it was probably already a derivative of the early root ᴱ√KIRISI as suggested by Christopher Tolkien (GL/27; LT2A/Cris Ilbranteloth). In the Name-list to The Fall of Gondolin Tolkien gave cris with the definition “a cleft, ravine, or narrow way of waters with high walls” (PE15/21), and in this period it typically appeared in this shorter form within names like G. Cris Ilbranteloth or G. Cris Thorn.
N. criss appeared in The Etymologies of the 1930s glossed “cleft, cut, slash” under the root ᴹ√KIRIS “cut” (Ety/KIRIS). It also appeared under the root ᴹ√KIR with the gloss “cleft, pass”, but this instance was deleted (EtyAC/KIR). S. criss “cleft” was mentioned in passing in Common Eldarin: Noun Structure (EVS2) from the early 1950s as derived from primitive ✶kirissi (PE21/80-81), and it was mentioned as a blending of roots in notes on Words, Phrases and Passages from the Lord of the Rings from the late 1950s or early 1960s as described above (PE17/87). Its use in names diminished over time, however, the only remnant in the final version of The Silmarillion being S. Crissaegrim (S/121).
Neo-Sindarin: In The Etymologies of the 1930s it seems this word was principally used as for a “cleft, cut, slash” independent of geography. I would assume the same is true for purposes of Neo-Sindarin, since criss is used only in a single geographic name in Tolkien’s later writings; S. cirith was use more broadly in geographic features. I would also assume it was a larger and more violent cut (a “gash” or “slash”) compared to S. rest for simple cuts.
An element in the (untranslated) name Orfalch Echor (S/239), but in earlier writings this word was translated “cleft, ravine”, and likely the Sindarin form of the word had a similar meaning.
Conceptual Development: The earliest iteration of this word was G. falc “cleft, gash; cleft, ravine, cliffs” from the Gnomish Lexicon of the 1910s (GL/33), where it was probably a derivative of the early root ᴱ√FḶKḶ “cleave, hew” as suggested by Christopher Tolkien (LT2A/Glorfalc; QL/38). It reappeared as ᴱN. falch “cleft, ravine” in Early Noldorin Word-lists of the 1920s as a derivative of primitive ✶spalk(w)e (PE13/143), and that’s the basis for the gloss given above.