The act of storing items that one would put in a fanny pack up the ass of ones self and/or in some cases the ass of another (sometimeswillingly, sometimes not)
Yourself:
"$7.50 for a beer?! Fuck that! I'm fanny packing one in!"
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.