Was Ernest Hemingway a bare knuckle fighter?

Not as such, no. But he was an enthusiastic fan of boxing, and wrote in his memoirs of 1920s Paris, 'A moveable feast', of his attempts in teaching Ezra Pound how to fight. In his prose he describes fights of all kinds - from boxing to bullfighting with considerable knowledge of the subject, and this is also reflected in his writing style; clipped and disciplined.