what does Sir Percy do to confirm the common assumption he is stupid?

Percy Blakeney appears to us first as the maligned husband of Lady Blakeney -- one of the richest, most fashionable men in England, but also a reputed dunce. He is built like a truck, apparently, with big shoulders and muscles, but this only adds to his reputation as a stupid stooge. He seems clueless about what is going on but is quite the opposite in reality.