In an interview with Complex, Stephen Le was asked what “based” meant.

“Based means being yourself. Not being scared of what people think about you. Not being afraid to do what you wanna do. Being positive. When I was younger, based was a negative term that meant like dopehead, or basehead. People used to make fun of me. They was like, ‘You’re based.’ They’d use it as a negative. And what I did was turn that negative into a positive. I started embracing it like, ‘Yeah, I’m based.’ I made it mine. I embedded it in my head. Based is positive.”
Thank you Based God.(Thank you Based God Stephen Le)

An exclamation made when your friends or family are teasing you to a point where you can't handle it anymore and a hissy fit is in order.

Derived from a YouTube user's famed outburst following Britney's lackluster performance at the 2007 VMA's.