A variation on the word "abscess." An abscess is a tender, inflamed mass-like collection of pus in the tissues of the body in response to infection. An assbscess is a collection of pus in the soft tissues of the gluteal region or posterior pelvis. It can be associated with the rectum, anus, gluteal muscles, skin, and/or soft tissues of the buttocks. It is posterior to the bladder and prostate gland in males and vagina in females. It does not involve the legs, and does not involve the genetalia.
A very tender infected mass in the buttocks near the anus would be an example of an assbscess.