Literally stands for "No My God". Translates more smoothly to "Oh my god no way". It is used to express extreme surprise at an unlikely event such as a hideous male scoring some Slam-A-Licious Babes. A first response
"I heard that Kartuigan is going out with Stacey now."
"NOMG, that's awesome. Way better than the Razorback he was dating before"