Tail
noun
Your butt tail is just your tail, though I wonder why they had to specify that it was your butt tail. Where else do you have a tail, I wonder?
The reading for this word is a bit of an exception. You kind of already know the reading for 尻 from the kun'yomi しり you learned already. Here it gets shortened to しっ. The reading for 尾 is totally different from what you learned, so here's a mnemonic to help you:
The animal with the best tail in the world is the polar (ぽ) bear. They're stubby and white and cute. Only polar bears have tails like that!
マリリンの猫の尻尾は、とてもフワフワです。
Marilyn's cat's tail is very fluffy.
だれも、タイソンに尻尾が生えていることには気づいていなかった。
No one had noticed that Tyson had a tail.
あの可愛い柴犬が、尻尾を振りながらコウイチに近づき、飛びついた。
That cute Shiba came towards Koichi wagging its tail and jumped on him.