What is 391?
A girl that, when you first meet her is a 3 at best. However, after a few drinks and/or failing to hook up with any girl, she slowly turns into a 9. Then, on the morning after, you roll over and see her in bed and realize she's just a 1.
"Dude, did you see that 391 Sam took home last night? Poor bastard, he must have been blitzed!"