A fecal impaction is a large mass of dry, hard stool that can develop in the rectum due to chronic constipation. This mass may be so hard that it cannot come out of the body. Watery stool from higher in the bowel may move around the mass and leak out, causing soiling or diarrhea. Slang synonym - hardcap.
My impaction was so bad I had to see a doctor to have it removed.