A word for “neck” in The Etymologies of the 1930s derived from primitive ᴹ✶yakta- under the root ᴹ√YAK (Ety/KEM), where the primitive k spirantalized and vocalized to i and the resulting diphthong ai became ae, sound changes that remain plausible in Sindarin as well.
Conceptual Development: There was a similar word G. gath “neck” in the Gnomish Lexicon of the 1910s from primitive ᴱ✶yatt- (GL/36), with the Gnomish sound change of initial y to g.
Neo-Sindarin: For purposes of Neo-Sindarin, I would use iaeth for “neck” as a joining between the head and torso, as opposed to lang as a passage from the mouth to stomach and lungs [including the “✱throat”] and S. ach referring the vertebrae in the neck [“✱(upper) spine”].
A noun in The Etymologies of the 1930s glossed “cavern” derived from the root ᴹ√GAT(H), an element in the name N. Doriath “Land of the Cave” (Ety/GAT(H)). In later writings S. Doriath was redefined as “Land of the Fence” with final element S. iath “fence” (WJ/370), so N. gath “cavern” was probably abandoned.