To nom something is to eat it with real enjoyment, often in a cute, playful way. You hear it from children, on the internet, and in silly videos: “nom nom nom” is the sound of happy eating. You can use it as a verb: “I’m going to nom this cake.” It is very informal. You would not put it in a school essay or a job email. Though actually, the letters N-O-M can stand for other things too — but this entry is only the eating one.