A clingy person, this could be a friend, a girlfriend, boyfriend... they cling onto you and demand attentionall the time as a barnacle clings to the bow of a ship.
The barnacle is a sexual manouver where the male spurts his love juice over a woman's bare chest. The woman must let the goo dry forming the 'barnacles'.
The woman then dresses and spends the day proud in the knowledge that someone loves her enough to barnacle her.