eat (one's) hat
A humorous action that one will allegedly take if something very unlikely happens. Kevin is always late, so if he actually shows up on time, I'll eat my hat.
eat your hat
mainly BRITISH, OLD-FASHIONEDIf you say that you will eat your hat if a particular thing happens, you mean that you do not believe that it will happen. I will eat my hat if the Social Democrats get in at the next general election. He has promised to eat his hat if he is wrong.