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.
Todd: What's wrong Billy?
Billy: My junk's all tangled up!
Todd: Ohh, sounds like you've got a tree hugger!