A noun from 1967 notes on the comparative, apparently meaning “hill, lump, clump, mass” and derived from the root √MBŎNO (PE17/93), where the um- developed from syllabic initial ṃ-. Tolkien introduced the root √MBŎNO to serve as a new basis for S. amon “hill”, motivated by his decision to give the root √AM “up” a new meaning: √AMA “addition, increase, plus”, so that it could serve as the basis for the intensive prefix am- (PE17/91).

Neo-Eldarin: I prefer Q. an- as the basis for intensives, but I think the word umbo(n) might be worth retaining in the more limited sense “lump, clump, mass”. For “hill”, however, I’d use the better attested Q. ambo.