A verb appearing as G. laitha- or laithra- “let slip, lose, mislay, forget” from the Gnomish Lexicon; it could also be used intransitively to mean “to be lost”, especially in the form laithra- (GL/52). Tolkien said it was the result of a mingling of two roots: ᴱ√lech and ᴱ√lith-, the former having to do with slippery things and the latter with time.
Neo-Sindarin: In 2018 I made an attempt to salvage this verb (my suggestion was ᴺS. laetha-), but this was rejected by the community. In a Discord conversation in 2022-04-16, Elaran proposed redefining this verb as lítha- “to let slip, lose, mislay; (intr.) to be lost” based on the later root √LIK “slip” which was recently published; by extension it can also mean “to forget”. I quickly adopted Elaran’s proposal as better than mine.
An adjective for “lost” appearing in its plural form laithi in Gnomish translations of the name of The Book of Lost Tales: G. i·band a·gwentin laithi or i band cwention laithi from The Gnomish Grammar (GG/11-12). It is clearly related to the verb G. laitha- “let slip, lose” from the Gnomish Lexicon (GL/52).