A word appearing as N. thinnas “shortness” in The Etymologies of the 1930s under the root ᴹ√STINTĀ, a noun form of N. thent “short” (Ety/STINTĀ), where the i survived in the noun because a-affection was inhibited. Contemporaneous notes on The Feanorian Alphabet had thinnas “shortening” as the name of a diacritic mark for short vowels in both the 1930s (PE22/31) and 1940s (PE23/22).

Neo-Sindarin: For purposes of Neo-Sindarin, I prefer to assume S. thent “short” was derived from ✶stentā as it was in 1957 (PE17/141). As such I would revise thinnas to ᴺS. thennas “shortness, shortening”; compare Ann-thennath “*Long-shorts” from The Lord of the Rings (LotR/193).