Eat one’s hat

A boast that is rarely carried out, first appears in Dickens Pickwick Papers (1836), “If I knew as little of life as that, I’d eat my hat.” The OED maintains the expression was originally Old Rowley’s hat. Old Rowley was the name of Charles II’s favourite horse and the name was later applied to the monarch himself who was often referred to as Old Rowley. The OED however gives no citation for this, nor does it explain why people would eat Old Rowley’s hat as opposed to any other.