The sound of someone eating typically a fat or obese person trying to chew far too much at once making the om nom nom nom sound.

"Om nom nom nom" is an onomatopoeia
Bill stuffs his face with cake, as he eats the sound resembles "om nom nom nom"
by Nicholas Busse December 8, 2007
Commentary generally given from a restaurant table adjacent to a table with one or more fatties regarding their eating habits. Hilarity generally ensues.
Carly: (Under her breath) Watch this guys, those whales over there are about to sit back and nom nom nom

Fatty: (Sits back) Nom nom nom
by thisisnotmyname8i758756 December 8, 2009
Delicious. A version of 'yum yum', usually typed not verbalised. Always in connection with cheap cake nobody with any discernment whatsoever would actually ingest, typically containing modest traces of actual cocoa and abusive amounts of artificial additives, and with a shelf-life of decades. Origins: onomatopoeia representing the sound of somebody pigging out in far too public a way. Does not help any conversation. Never use this expression.
"Hey guys, Asda cakes are half price today! #winning".
"Cake! Nom nom nom".
by Frou-Frou December 23, 2018