When you're showering with a girl and she's giving you a blowjob, you shampoo your hair and rinse the bubbles on to her hair. She then uses those bubbles to wash her hair, all the while still sucking the dick. This is called a Head & Shoulders.
I just got a Head & Shoulders last night. I've never felt so dirty and clean at the same time.
When you are being masturbated by a partner, and said parter shoves a travel sized bottle of head and shoulders up your ass. Typically done in the backseat of a car in the middle of the day.
Guy 1: Man, Laura invited me to a Head & Shoulders Party last weekend. It was awesome! We had it right in front oh her church!
Guy 2: Ew.