Another word for erection. Originated from getting boners in school everyday around 9:30. You can walk around with it down your leg to show everyone, or you can tuck it under your ass if you're ashamed of it. When your boner is limp and no longer of value to your sexy ass teacher, it is called a "9:31".
When the fly ass whore sat on my dick, I felt like it was 9:30, so she fucked my dick 'til I busted baby juice.