This word is unknown to the OED, but the invaluable on-line Urban Dictionary tells us — at some length and with a lot of repetition — that a nom is the sound one makes when eating something, and that to nom is to eat.
(Just search for nom in Google Images to see hundreds of examples.)
What, I wonder, could be the etymology of this term?
In Jamaican (and other Caribbean) Creoles there is a word nyam, which can be a noun meaning ‘food’ or a verb meaning ‘to eat (particularly in a crude manner)’. The word has now made it to the OED, which summarizes its probable etymology as
a West African language (e.g. Wolof ɲam to eat or Fula ɲama to eat), although perhaps simply imitative.
Here is the beginning of the more detailed treatment available in Cassidy & Le Page’s Dictionary of Jamaican English (CUP, second edition 1980).
Although foreign-language a is mapped onto the TRAP vowel in British and Caribbean English, in AmE it could well be mapped onto the LOT vowel. (Compare BrE kebab kɪˈbæb, in AmE also kabob, from Arabic etc. kabāb.) The non-English initial palatal nasal ɲ might well have been replaced by the familiar n. Hence a source form ɲam might have become AmE nɑm, which would naturally be spelt nom.
I haven’t got access here to a word list of Gullah (or indeed of AAVE) to check for n(y)am, n(y)om there, but perhaps one of my readers has. If found, that might clinch the matter.
In short, I wonder if nom is a doublet of nyam and is thereby one of the rather few words of English taken from an African language. Or is it really “simply imitative”?