an excuse made by desperate girls after having a sexual encounter with an ugly boy. the "3-9-1" describes their level of hottness on a scale from one to ten.
3: when you first meet them
9: the desperate point during the encounter
1: when you see them after
girl 1: did you do the nasty with him?
girl 2: eww yes.
girl 1: actually?
girl 2: yaa, i had a total 3-9-1