What is the source of the allusion in the sentence below?
Most of the people who knew Mitch considered him to be an unrepentant Casanova.

 - Shakespeare
 - history
The source of the allusion Casanova is history.
The autobiography of Giovanni Giacomo Casanova, an eighteenth-century Italian adventurer, details and perhaps exaggerates his amorous adventures and success with women.
The allusion Casanova means a womanizer.