Short answers, pulled from the story.
Mozart wrote scatological humour into at least 39 of his personal letters, nearly all addressed to close family members, and composed several scatological canons that were published only with bowdlerized lyrics after his death. Musicologist David Schroeder argues this reflects the norms of 18th-century popular culture rather than any personal pathology.
The Basle letters are a group of letters Mozart sent to his cousin Maria Anna Thekla Mozart, described by musicologist Maynard Solomon as a probable love interest. Written after Mozart spent two weeks with her in Augsburg, the letters combine wordplay, sexual references, and scatological verse. Mozart's widow Constanze described them in 1798 as "full of wit" but "in dubious taste".
No credible medical consensus supports this diagnosis. Benjamin Simkin proposed it in the 1990s based on the scatological letters and accounts of physical tics, and the claim spread widely in newspapers and online. German psychiatrist Thomas Kammer concluded in 2007 that Tourette syndrome is "an inventive but implausible diagnosis", and no Tourette's expert or organization has endorsed it.
"Leck mich im Arsch", catalogued as K. 231, is a canon for six voices composed by Mozart some time in the 1780s. Its title is a standard German vulgarism, euphemistically known as the Swabian salute. It was first published after Mozart's death under the bowdlerized title "Lass froh uns sein" ("Let us be joyful").
Scholars point to several explanations: scatological humour was widespread in 18th-century German popular culture, many of the phrases Mozart used were pre-existing folk expressions rather than his own invention, and the humour was shared across his entire family, with his mother Anna Maria and father Leopold also writing scatological passages. Musicologist David Schroeder argues the vulgarity carried political overtones as a counterpoint to aristocratic refinement.
Freud was approached by the Austrian writer Stefan Zweig, who sent him copies of the Basle letters with a suggestion that they revealed elements of "infantilism and coprophilia" worth studying. Freud apparently declined to pursue the analysis.