http://www.iep.utm.edu/cynics/

If Antisthenes was not the first Cynic by name, then the origin of the appellation falls to Diogenes of Sinope, an individual well known for dog-like behavior. As such, the term may have begun as an insult referring to Diogenes’ style of life, especially his proclivity to perform all of his activities in public. Shamelessness, which allowed Diogenes to use any space for any purpose, was primary in the invention of “Diogenes the Dog.”