PubLic hair is pubic hair that has been found in a public or communal place such as a bathroom toilet or bathroom shower/tub. The hair may either have fallen out or be the remains of a pubic-shaving session.
Mark: Sick bro, you live in a house with four other guys! You need to learn to remove the evidence of your public hair! I don't want to sit on that shit while I'm sitting on the toilet!
When someone's pubic hair becomes dislodged during a visit to the bathroom and remains on the toilet seat or bowl for the next person using the restroom to see, resulting in the transition of your private's hair to a public hair.
Why is it so hard for people to clean their pubes off the toilet when they're done? I don't need to see their public hairs.