Mostly used by immature girls that don't actually give a fuck about what you're doing...especially if they couldn't even type out something longer then a 3 letter response.
2nd in pathetic to "lol", people that use "nmu" in a serious form deserve to be forever deprived from their cell phones and computers.
If a response as pathetic as "nmu" is used as a response, you should not even respond in the first place.
Guy: Wow that's a fucking pathetic response
Girl: Soorrryyy I'm (insert bullshit excuse here)