Pilonidal sinus is a midline post anal sinus commonly seen in young adults usually without any communication with rectum and anal canal. Pilus means hair. Nidus means nest, so Pilonidal sinus means Nest with hair inside. where hair is invaginated in subcutaneous tissue as a Bulbous Diverticula with branching and bifurcating from side. Pilonidal sinus is common in the gluteal cleft. While sitting, the buttocks move and the hair is broken off by friction. The hair collects in gluteal cleft.