A man or a woman who's dating someone WAAAAY older than he/she is. Most likely someone with children with his/her husband/wife away. This term came to life through comparing an old tree with a partner the person is having.
Guy: Dude! You're a fucking Tree Hugger! That bitch has 2 kids!
Guy 2: Aww cmon man, the ho's husband's out of town for two weeks! Let's live life to the fullest!
Girl: I'm such a Tree Hugger!
Girl 2: Yes you are.