A root given as maƀ or mam in the Gnomish Lexicon of the 1910s glossed “something nice”, with derivatives like G. mab(a) “mother”, G. mam “grandmother”, G. mav- “like”, and G. mavri “appetite” (GL/57). There are no signs of this root in Tolkien’s later writing, but I think it is worth positing a Neo-Sindarin root ᴺ√MAB to salvage some of these early words.
Quenya
haruni
noun. grandmother
A noun appearing as mam “grandmother, mother” in the Gnomish Lexicon, derived from ᴱ√maƀ or mam “something nice” (GL/57).
Neo-Sindarin: As we have no other attested alternatives, I would use ᴺS. mam for “grandmother” in Neo-Sindarin, based on a Neo-Root ᴺ√MAB (from primitive ✱mamba), but would not use it as “mother” for which we have plenty of later words.