Spinny is slang for is adding tobacco to your weed so you get head-spins and so that your cone or joint is smoother on your throat. Usually done by pulling apart a cigarette and cutting it up with your marijuana.
Slang mainly used in Australia.
Person 1: Oi bro add some spinny into the cone?
Person 2: Got any smokes on you then?
Person 1: Yeah, pull it apart and cut it up then man.