Stands for "Not Being Lez". Similar to the frequently used phrase "No Homo", but specifically geared towards women. If a woman says something that sounds unintentionally sexual to another woman, saying "NBL" or "Not Being Lez" afterwards is a way of making it clear that it was not intended to sound sexual. Its is also used ironically in the gay community.
Shyla - "OMG your cat is so soft! I just love your pussy!! ..NBL"
Adrianne - "Sure.."