In The Etymologies of the 1930s there were two roots: unstrengthened ᴹ√MER “wish, desire, want” and strengthened ᴹ√MBER, unglossed but with derivatives like ᴹQ. meren(de)/N. bereth “feast, festival” and ᴹQ. merya/N. beren “festive, gay, joyous” (Ety/MBER, MER). It is not clear whether Tolkien intended these roots to be related; the entry for ᴹ√MBER first gave the root as ᴹ√MER with N. mereth and meren, only to change it to ᴹ√MBER, N. bereth and beren when ᴹ√MER “wish, desire, want” was introduced (Ety/MBER; EtyAC/MBER).

In both earlier and contemporaneous Silmarillion drafts of the 1930s, however, Tolkien used N. Mereth Aderthad for “Feast of Reuniting” (SM/329; LR/126, 253) and continued to use S. Mereth Aderthad in the Silmarillion narratives of the 1950s and 60s (S/113; WJ/34). Possibly also related was the revision of the name Beril “Rose” to S. Meril in the unpublished epilogue to The Lord of the Ring written from the end of the 1940s (SD/117; SD/126). Conversely, {√MED >>} √MER “wish for, want” appeared in the Quenya Verbal System (QVS) from 1948 (PE22/102; 103 note #23) and Q. mer- “hope” appeared in the so-called “Merin Sentence”: Q. merin sa haryalyë alassë nó vanyalyë Ambarello “I hope that you have happiness before you pass from the world”, of unknown date but certainly written after the 1st edition of The Lord of the Rings (MS).

Thus it seems Tolkien merged ᴹ√MBER back into ᴹ√MER, though the meaning of the combined root isn’t clear given its various derivatives with senses like “feast”, “rose” and “wish, hope”. The last of these probably remained the core meaning, perhaps with “festive” extrapolated from “hopeful”.