This is obtained by wearing all the possible inventory increasing fanny packs. These can be found at any point randomly during the game, and sometimes bought from the store in Town. Each bag will increase your carrying capacity by +3.
The three items to lookout for are:
-Bum Bag
-Fanny Pack
-Side Slung Fanny Pack
I found these pretty early in the dormitories. Floor 4 of the first cooler.