Stands for Not In Real Life friend. Usually an example of an internet girlfriend
Friend 1: Ooooh, this is a steamy conversation, who's this chick?
Friend 2: Oh, she's my new nirlfriend.
A contraction of not girlfriend. A woman who you are sleeping with but the line is blurring and at least one of you doesn't want to call it a relationship.
Are you seeing your nirlfriend at the weekend?