To fart. Academics believe that the expression comes from when you fart so hard and so loudly that you feel like you have just ripped/teared you ass. However, you don't necessarily need to rip your ass to "let it rip".
Dude1: Duuude... you are about to get GASSED!!!! >:D
Dude2: Whatever... let it rip. |-/
Dude1: *fart* XD