A noun appearing as N. sûth “draught” in The Etymologies of the 1930s derived from the root ᴹ√SUK “drink”, likely from primitive *suktō given its Quenya cognate ᴹQ. suhto (Ety/SUK). It is thus likely that the combination ukt vocalized to ūth, as it did for similar words in The Etymologies such as N. lhûtha- “enchant” vs. ᴹQ. luhta- under the root ᴹ√LUK (Ety/LUK).

Conceptual Development: The Gnomish Lexicon of the 1910s instead had G. suith “a drink, a draught” (GL/68) from the early root ᴱ√SOKO (QL/85), representing a different vocalization: okt vocalizing to oith (HPG/§2.6) and then oi becoming ui (PE15/13). It seems the phonetic developments in The Etymologies of that late 1930s were different, but in notes on The Feanorian Alphabet from this same period, Tolkien said [ui] was the result of vocalized u + χ (PE22/40), and indeed that seems to be the phonetic developments in later Sindarin as well, given words like S. nuitha- from primitive ✶nuktā- (WJ/413).

Neo-Sindarin: For purposes of Neo-Sindarin, I’d use ᴺS. suith for “draught, a drink”, a form I saw first suggested by David Salo in his book Gateway to Sindarin (GS/321).