A tree hugger is a a temporary discomfort of the male genitalia which occurs when one's moist scrotum "wraps" itself around the less moist shaft of the
penis, providing a keen grip. In doing so, the testicals separate from one another and are displaced by the
penis. This displacement causes significant tension on the sides of the shaft, as if the testicals were "hugging" the
penis.
So in other words, it's when your sweaty balls wrap around your cock, it's not cool.