A word appearing as G. cuith in the Gnomish Lexicon of the 1910s with glosses “life, the vital principle” and “a living body”; Tolkien specified that it was never used for “a lifetime”, which was instead G. cuil (GL/27). It was likely based on the early root ᴱ√KOẎO “have life” as suggested by Christopher Tolkien (LT1A/Koivië-néni).

Neo-Sindarin: For purposes of Neo-Sindarin, I’d retain ᴺS. cuith as “life, the vital principle” in reference to the biological process of life and being alive. For “life” in its more ordinary sense I’d use [N.] cuil.