Stands for "No Sex Boy Friend"
A guy in your life who calls you every hour, gets mad when you don't call him back, gets all emotional about your relationship but have absolutely no SEX with each other at all!!! aka. your best guy friend in the entire world or your boy friend without any perks, opposite of friends with benefits
"Jonee my NSBF just flipped out on me on facebook because I didn't call him back last night"