A derivative of the formally dominant "on {X thing}" , it makes similar reference and is short for "on shaman".
Bro 1: "Hey man, you seem pretty chill, what do you believe in mann?"
Bro 2: "spiritually you mean?
Bro 1: "sure yeah, like religion..."
Bro 2: "uhhh I just believe in spreading love"
Bro 1: "wait what? that's not a religion man... cmon tell me"
Bro 2: "I'm telling you it's that simple, on sha brotha, on sha"
Bro 1: "whatever man.... If you say so"
Bro 2: "....."