Abbreviation for Oh My Fred and George. Predominantly used by fans of J.K.Rowling's bestseller series Harry Potter, it is a censored version of OMFG that is used to express approval or awesomeness.
A: Wannaslytherin my chamber of secrets?
B: OMFnG!
A: Ouch.
B: No, OMF N G.
A: Ohh...eyyyyyyy. ;)