A noun from 1967 notes on the comparative, glossed “a pile or mass of rock or earth” (PE17/93). It appeared underneath the root √MBŎNO, but was almost certainly actually derived from √MAB “lump, mass” appearing earlier in the same notes (PE17/90). Both these roots were considered as a new basis for the words Q. ambo and S. amon “hill” when Tolkien repurposed the root √AM “up” as √AMA “addition, increase, plus” in order to serve as a new basis for Quenya comparative forms, with √MAB >> √MBŎNO.
Neo-Eldarin: Given the evolution of its roots, this Sindarin word is pretty dubious, but I don’t have a good alternative for it at the moment.
n. a pile or mass of rock or earth.