A reference to a flamer, or homosexual. A flamestacker is different from a flamer since they actually stack flames, meaning that they create homosexuals as well as being one. They are the scouts for converting straight and bisexuals to full-blown homosexuals. They are the zenith of homosexuals.
Diesel: "Dude, Devin is such a flamestacker. Look at him over there trying to stack those guys over there."
Chimptits: "Uhhh, yea, thats some gross ass shit. I knew he was gay, but i didnt know he was a flamestacker too!"