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 one's hat
Fig. a phrase telling the kind of thing that one would do if a very unlikely event really happens. If we get there on time, I'll eat my hat. I'll eat my hat if you get a raise. He said he'd eat his hat if she got elected.