A fat pantry is a pantry (or shelves, cupboards or whatever) where you keep a large stock of foods that you regularly eat. I've heard some call a fat pantry their own personal store. The idea is that you stock up when items are on sale (or regularly if it's something that never goes on sale) so that you always have several of an item on hand. Then you can shop out of your own pantry.
This is the way my mother shopped. She could make hundreds of recipes on the spot without having to go to the store because she had shelves full of things that she regularly used. In fact, I only remember a handful of times that she had to purposefully pick up something at the store for a recipe. She just always had one of everything on hand.
Now, I use the same method in my own home. It's taken some time to get a well-stocked or fat pantry. But I try to have some of everything on hand.
There are so many great things about using this method. First, I am able to always save money on my groceries -- I rarely have to run to the store (and use gas) to get an item that is likely not on sale. I buy a lot of my foods on sale -- and when it is discounted I stock up. The second great thing about this method is that I am never locked into a menu. Planned for enchiladas, but really in the mood for wild-rice soup? No problem - ingredients for both are usually on hand. I can make the majority of my favorite recipes right now because I have all of the ingredients in my "fat" pantry.
Finally, if you think about it, a fat pantry can also be a three-month supply in disguise. A three-month supply is an important part of a good family home storage plan. I recommend following the family home storage plan outlined by The Church of Jesus Christ of Latter-day Saints. In their plan, a three-month supply is just one of four important steps in gathering a complete home storage. By having a full fat pantry (three-month's worth) you can complete this part of the storage plan. You also gain some of the promised blessings of peace by knowing that your family is better prepared.
Do you have a fat pantry? How do you make it work for your family?