A noun in Quenya Notes from 1957 (QN) derived from primitive ✶(s)lōkō “reptile, snake, worm” based on the root √LOK “bend, loop”, so presumably having a similar meaning (PE17/160). Christopher Tolkien also had lhûg in The Silmarillion appendix, but gave it the glosses “snake, serpent” (SA/lok).
Conceptual Development: In the Gnomish Lexicon of the 1910s, Tolkien had G. ulug “dragon” (GL/74), and in the contemporaneous Official Name List for the Lost Tales and the Name-list to the Fall of Gondolin he had G. lug or lûg “snake” (PE13/105; PE15/28). In The Etymologies of the 1930s, Tolkien had N. lhûg “dragon” under the root ᴹ√LOK “great serpent, dragon” (Ety/LOK), where initial l was unvoiced to lh as was usual in Nodorin. This was no longer true of Sindarin of the 1950s and 60s, so Sindarin lhûg required a primitive from ✶slōk- and a corresponding Quenya form of hlócë.
Neo-Sindarin: Given the breadth of its glosses, I would assume this word can apply to any sinuous reptilian creature with or without legs, including lizards, snakes and dragons.
_ n. Zoo. _Q. hlóke, lóke. (s)lōkō << (s)lōkā.