An abbreviation for those who sleep around with multiple random people. It stands for "Fuck And Run". These generally apply to those who like having sex but don't want to stick around to deal with the mess in the morning.
Leum : "Okay, so I hear Leah's mad ass hot and a cougar in bed."
Alex : "So, what holding you, man.. go tame that cougar!"
Leum : "Yeah, but the words out that she's too clingy, what if she glues herself to my ass for life?"
Alex : "Oh well, then you gotta FNR on that bitch!!"