"Nom nom nom" is essentially an onomatopoeia for the chewing noise made by the Cookie Monster from Sesame Street. It is a phrase used to indicate hunger, chewing, or pleasure from eating food. For instance, if someone were to respond to a picture of a plate of food online by leaving a comment that says "nom nom nom," it would indicate that that person thinks the food looks delicious and he or she would like to eat it. The spelling and spacing may change depending on the source, producing such variations as "om nom nom" or simply "nomnomnom."
While "nom nom nom" was mostly used to describe the sound of eating cookies at first, it has branched widely off of that path. Now, the phrase can indicate chewing or eating any number of foods or objects. Additionally, the eater is no longer just the Cookie Monster, but can also be any person, animal, or even artfully depicted inanimate objects.
This phrase is generally only used in very informal settings. While the phrase is used widely on the Internet, many people find it awkward or inappropriate to use in real life situations. It may be attributed to the form of mutating Internet language known as "lolspeak." Sometimes, the phrase is used as a comedic insult to describe a person who is tearing into his or her food with enthusiasm. For example, it would likely be very rude if an individual were to tease his or her boss by saying "nom nom nom" as he eats his food quickly during a break from work.
Sometimes "nom nom nom" is used as a caption on various pictures on the Internet. Frequently, the subject of the pictures is a cute animal munching on some food or another object. Other times, the caption describes a picture of an animal or person caught on camera taking an abnormally large bite out of something. If a photograph of an inanimate object or something in the environment appears to be biting or swallowing something else, the picture may end up in forums or other Internet locations digitally altered to have cartoonish teeth and a mouth added on, captioned with "nom nom nom!" To add even more personification, cartoon eyes, eyebrows, and other facial features are all commonly added in an attempt to convey an emotion such as anger or delight from the eater toward the object being eaten.