According to Diogenes Laërtius’ third-century Lives and Opinions of the Eminent Philosophers, Plato was applauded for his definition of man as a featherless biped, so Diogenes the Cynic “plucked the feathers from a cock, brought it to Plato’s school, and said, ‘Here is Plato’s man.’ ” When asked about the origin of his epithet, cynic deriving from the Greek word for dog, Diogenes replied that it was given to him because he “fawns upon those who give him anything and barks at those who give him nothing.”

Borrowed from: https://www.laphamsquarterly.org/animals/miscellany/plato-and-diogenes-debate-featherless-bipeds