The substance in between a man's ass cheeks that creates skid marks
. Also known as butt butter
, gack in the crack is usually formed as a result of several components put together. These are:
1) A hairy ass
2) A moist defecation followed by clean up with cheap TP
3) Physical exertion that causes the hairy ass to perspire
Mike took a hurried dump in the portajohn outside the stadium and then went back to tailgating. It was a long walk back to the pickup and the hot weather made him sweat. By the time he got back to the beer cooler he had some gack in the crack.