I found this story on @HistoryWeird's blog here. The original source was the "Diary of Maria Anna Mozart", August 19th 1780; May 29th 1775.
For those of you who have seen Amadeus, Mozart was probably very much like how he is represented in that film. There is ample evidence to show that he loved a bit of vulgar buffoonery and was not a figure of dignity and solemnity, despite the genius of his music.
For example, he wrote a song entitled "Leck mich im Arsch" (literally "Lick me in the arse"), which was very likely a party piece for his friends.
He also liked to mess with his sister, as any mischievous brother would:
"In August 1780 Wolfgang Mozart, then aged 24, happened upon his sister Maria Anna’s diary. Pretending to be her, he wrote the following entry:
'"About shitting my humble self, an arse, a break, again an arse and finally a nose, in the church, staying at home due to the whistle in the arse, whistle not a bad tune for me in my arse. In the afternoon Katherine stopped by and also Mr Fox-tail, whom I afterwards licked in the arse. O, delicious arse!'"
The hell does that even mean? Musical genius he may have been, but lyrical wordsmith? Not so much.
"This was not the first time Mozart had written in his sister’s diary without her permission. In May 1775 Maria Anna mentioned attending a concert in the city hall, featuring a female singer. Beneath her entry, Wolfgang scrawled:
Somebody get Mozart a dictionary. He really needs to learn some more interesting words. He's kind of a one-trick-pony when it comes to writing vulgarities. Arse arseity arse arse arse aaaaaand bum.