A pilonidal (pie-low-NIE-dul) cyst is an unusual pocket in the skin that usually contains hair and skin debris. The cyst is almost always near the tailbone at the top of the buttocks.

Pilonidal cysts usually occur when hair punctures the skin and then becomes embedded. If a pilonidal cyst becomes infected, it can be very painful. The cyst can be drained through a small cut in the skin. Sometimes, surgery is needed.

Pilonidal cysts are most common in young adult males, and the problem tends to recur. People who sit for long periods of time are at higher risk of developing pilonidal cysts.

https://www.mayoclinic.org/diseases-conditions/pilonidal-cyst/symptoms-causes/syc-20376329