Here's the definition of one from the Mayo Clinic:
'Definition By Mayo Clinic Staff
A pilonidal (pie-low-NIE-dul) cyst is an abnormal pocket in the skin that usually contains hair and skin debris. A pilonidal cyst is almost always located near the tailbone at the top of the cleft of the buttocks. These cysts typically occur when hair punctures the skin and then becomes embedded. If a pilonidal cyst becomes infected, the resulting abscess is often extremely painful. The cyst can be drained through a small incision or removed surgically. Pilonidal cysts most commonly occur in young men, and the problem has a tendency to recur. People who sit for prolonged periods of time, such as truck drivers, are at higher risk of developing a pilonidal cyst.'
So, prolonged sitting can be a cause of the subject cyst, such as being a cavalryman or a horse artilleryman, or a train driver/conducteur.
I've had one and I rode for years. It had to be surgically removed and was at the end of the tailbone. It was a real pain.