The name of the special bread used by Elves during travel, more literally “waybread”.

Conceptual Development: This word appeared as N. lembas “waybread” in a marginal entry to The Etymologies of the 1930s derived from the root ᴹ√LEN “(road?), way” [the first gloss is unclear], a variant of ᴹ√LED “go, fare, travel” (EtyAC/LEN). It second element was probably N. bast “bread” (Ety/MBAS). In the narratives themselves, the word lembas appeared in early drafts of Book 2, Chapter 8 “Farewell to Lórien” from The Lord of the Rings (TI/279), which is where it first appeared in the published version as well.

Possible Etymology: Although its second element was probably originally bast “bread”, it is not clear why this word was lembas and not lembast; Noldorin compounds usually retained final st, as in Orchrist and Camlost. The reduction of the st in lembas could be a remnant of earlier ideas; compare it to G. losbas “ryebread” = G. losc + G. bast from the Gnomish Lexicon of the 1910s (GL/22, 54).

Nevertheless, Tolkien considered the etymology of lembas somewhat problematic in his later writings. In notes from the 1950s he derived it from OS. lenn-mbass “journey-bread” (PM/404). In notes on Words, Phrases and Passages from the Lord of the Rings from the late 1950s or early 1960s Tolkien derived lembas from lendbas where its initial element was lenda “journey” based on the root √LEN or √LED but he did not explain the second element (PE17/60). In notes from the mid-1960s Tolkien wrote:

lembas “waybread”. This seems meant to be associated with Primitive Eldarin stems *LED “go” and *MBAS “bake” but is not readily derived from them according to Sindarin developments. Something like *leðbast would be expected (PE17/51).

In these notes, Tolkien initially derived the word from ledme-mbasta but struggled to explain the reduction of st. He then started over, saying:

Simplify this thus: Assume a Primitive Eldarin derivation *mbassē, “(baked) bread” ... Evidently lembas is an old compound < led(e)mbasse “bread taken on leaving home (for a long journey)”, the first element being the bare verbal stem. ... lembas would be a reasonable Sindarin development in an obscured compound though leðbas would be expected if the stem √LED had remained in common use (PE17/52).

This revised etymology is more-or-less compatible with the 1950s derivation from OS. lenn-mbass, which is the derivation Christopher Tolkien presented in the Silmarillion Index (SI/lembas).