YOU SAID:
You are such a fat donkey
INTO JAPANESE
あなたはとても太ったロバです
BACK INTO ENGLISH
you are a very fat ass
INTO JAPANESE
あなたはとても太ったお尻です
BACK INTO ENGLISH
you have a very fat ass
INTO JAPANESE
あなたはとても太ったお尻を持っています
BACK INTO ENGLISH
you have a very fat ass
That didn't even make that much sense in English.