“Slang, indicating eating. Commonly used is lolcats and similar image macros. It usually translates as, “I am absorbed in eating this” or “OMG this is awesome eats, I’m eating it!”.

The phrase is suspected to have it’s origin in the sounds the Sesame Street character Cookie Monster when eating cookies. In the contemporary lolcat usage, the “om” is often omitted, especially when in the middle of the act of chewing as opposed to taking the first bite and then chewing.

The precedent, original usage includes adding eyes, eyebrows and/or sharp teeth over photorealistic images with large, mouth-like subject matter, indicating a “swallowing up” or other like-failure.”

via RocketBoom

see Also – http://www.omnomnomnom.com/