A fatbagger is a bargain hunter buying large amounts of products (often expensive brands and products) when they are on sale. From the meaning of filling a big shopping bag with products making the bag a fat bag.
A person who goes on a simple hike but proceeds to wear a fanny pack, or fag bag, and claims that it is a "practical hiking gear satchel" and has unnecessary energy bars for the hike.
"Dude, the hike is only a mile long to the lighthouse, why bring the fag bag?"
"Hey, you can never be too prepared! I also have a compass and flashlight."
"You're such a fagbagger."