If I had ever even vaguely thought about it, I might have assumed that the sound/word nom/nom nom/nomnom etc had been created by the writers of Sesame Street or improvised by Cookie Monster’s actor (who I have just discovered was originally Frank Oz). Certainly om nom or om nom nom is so associated with it/him that Wikipedia redirects searches for that, to its page for Cookie Monster. (The video of Cookie Monster I found seems to use nom and nyom.)
In my last post I mentioned that I am listening to/reading James Joyce’s Ulysses. Surprise – nyumnyum appears there. Just after the conversation I wrote about in my last post, Leopold Bloom remembers “Old Mrs Thornton” (?a midwife/child nurse) feeding babies “pap” (soft food, maybe bread soaked in milk). “O, that’s nyumnyum”.
Not surprisingly, nom, nyom, num, nyum by whatever spelling and combination, doesn’t appear in any formal dictionary Google can find, given the informal spoken variability of it. It may have appeared in writing before Joyce, and certainly pre-dates him as a sound/spoken word.
Dictionary.com dates yum-yum from 1880-85, which means it was hot off the press when Gilbert and Sullivan used it in The Mikado. Surprisingly, yummy dates from later, 1925-30.
Putting aside the results for nom and num (which have other uses), Google Ngrams shows nom nom and num num clearly ahead of nyom, nyum, nomnom, nyumnyum, nyum nyum and numnum, with no results for nyom nyom and nyomnyom (both of which show on a general Google search). I guess how you write it largely depends on how you say it, but it’s likely to clear from the context and no spelling is likely to have vociferous advocates telling you that you’re a nomskull for spelling it like that.
Nomskulls actually exist – they are skull-shaped cupcake mo(u)lds, I’ve just found out. Throwaway silly jokes have a habit of sidetracking me.
Numskull has been the more common spelling for most of its history, until numbskull overtook it in the mid-1980s. Numbskull is literally numb + skull but num (by itself) is the older form (from Middle English nome, from an Old English word meaning taken, stolen (in this case, feeling)). The b was later added to conform with comb (which had a b in Old English) and limb (which didn’t in Old English = arm, leg, wing, branch, but did in Latin, limbus = edge of disk of sun or moon, graduated edge of quadrant). (Dictionary.com)
Having mentioned Frank Oz, a random memory. Many years ago I watched The empire strikes back in a cinema. When Yoda appeared a young boy in front of me whispered excitedly to his father “Kermit …!”. I wanted to whisper “No … Fozzie!”