I'll eat my hat!
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.
I’ll eat my ˈhat!(spoken) used to say that you think something is very unlikely to happen: They’re always late — if they get here before eight o’clock, I’ll eat my hat.
See also: eat