The act of storing items that one would put in a fanny pack up the ass of onesself and/or in some cases the ass of another (sometimes willingly, sometimes not)
Yourself:
"$7.50 for a beer?! Fuck that! I'm fanny packing one in!"
Another:
Jon: "What do you mean you lost your car keys?"
Jamie: "Me and my girlfriend were fanny packing last night and I haven't seen 'em since."
The act of storing a cell phone with a google map app in your fanny pack while having sex. The app keeps track of speed, distance and duration for you.