A cross between 2pac the artist and packing heat. It means to Be carrying a gun for yourself and a gun for a friend, hence the name 2pac'kin heat. Tupac Shakur (2pac) used to do this for his close friends.
(a black gangsta approaches a fellow gang member)
First Nigga: "2pac'kin heat ?"
Second Nigga "Nah, fool, gonna pop some cracka's on ma own.