An abbreviation of "not much" or "nothing much". It is most commonly used in instant messaging conversations as an easy and thoughtless answer to the question what's up
Some people also use it as an abbreviation for "nevermind", but "nvm" is more popular.
Typical start of an MSN conversation:
Jill: nm u?
Jack: (starts talking about something)
Free Daily Email
Emails are sent from firstname.lastname@example.org. We'll never spam you.