A lump on the tailbone may be due to several reasons: 1. There may be a condition of spondylolisthesis. This is a genetic condition that is caused by congenital genetic factors, resulting in spina bifida and a bulging spinal membrane, which is seen as a swelling on the surface of the body that protrudes from the skin and fluctuates when pressure is applied. In some patients, there is also skin pigmentation or hair growth. 2. This condition may be caused by subcutaneous occupational disease, such as lipoma, which is most common here. 3. This condition may be caused by old injuries, such as old fractures or soft tissue blunt contusions, and after recovery, there may be excessive fractures or scarring here, which can 4. This condition is likely to be caused by skin lesions here. The most common is sebaceous cysts, which are often exposed to high temperatures and humidity, and are often squeezed and rubbed, which may cause skin infections and lead to these diseases.