a spank mag is like all these visions in your head of hot action you've seen, or people you know or have seen that you can totally picture naked or in some sort of sexual fantasy and this is what you choke your chicken too.
every guy's done it but this is the actual word for it
spank mag is commonly used if you see a hot chick you yell "SPANK MAG!" this means u can totally picture this chick naked and totally wanna wack to it
(hot chick walks by) "Dude, this chick is TOTALLY goin in my spank mag."