Technically any fold of fat of a morbidly obese person, but in common usage one that harbors foul-smelling excretions. These can be simple body odor, areas that were too hard to reach during showering, or areas that have become fouled through elimination, the impossibility of spreading the cheeks, or constrained by the shape of a toilet seat (the latter two causing a "muddy fatcrack"). The word can also be used to describe odors caused by a fatcrack - in this case 'fatcrack' is most often experienced when the fat person moves, releasing a bolus of odor, or 'fatcrack'.
1. "Damn, that lady on the bus had some serious fatcrack going on."
2. "What's that smell? - Oh, that's Ruth, she's got a muddy fatcrack".
A extra large crack, as in your butt crack of course. A crack or but, if you may, so large large that it takes up an excess
amount of room.
Joey: She has a huge butt
Max: Naw she has a huge fatcrack
That man who just sat down has such a large fatcrack he took up 3 seats.