A shorter phrase for tentacle porn (hentai anime like Urotsukidoji or Cool Devices), usually said in a sarcastic, negative context by other anime otaku who don't care for it.
Shoujo Otaku: wanna watch Marmalade Boy again?
Henjin Otaku: nah, i rented this Uuu... rats... oookie ... um, this video. i wanna watch that instead.
SO: eh? what's it about?
HO: hmm, schoolgirls... demons... looks kinda ecchi.
SO: (looks at video box) awesome. tree rape. i gotta go now. (sprints away)
Tree rape doesn't always involve trees, but it's based on the same premise as tentacle rape - some poor demon-possessed creature's appendages come to life and do various things to innocent 16-year-old schoolgirls.
It can get pretty nauseating. I recommend trying to sit through it at least once in your life, just to prove it exists. Then watch a marathon of all 96 episodes of Maison Ikkoku to get the burned images out of your head and remember that: No, anime is not all porn, it's only the small subgenre that happened to get huge stateside.