This is a pun on 'callously' which means in a way that shows an insensitive and cruel disregard for others; unfeelingly, but it's also used because the alternate spelling of 'callus' means a thickened and hardened part of the skin or soft tissue, especially in an area that has been subjected to friction, like you'd develop if your were rowing
' ' Rowing hurts my hands , ' ' said Tom callously .