A word appearing as ᴱN. {cuilborn >>} cuilvorn “lifetime”, a combination of cuil “life” and born “age” (PE13/141). In the Gnomish Lexicon of the 1910s it was G. cuilborn “lifetime” with the same elements (GL/27).

Neo-Sindarin: Since [N.] cuil “life” appears in Tolkien’s later writing, I’d retain ᴺS. cuilvorn “lifetime” for purposes of Neo-Sindarin, and assume the second element was based on the root ᴹ√BOR(ON) “endure”, hence = “*life’s endurance”.