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.
eat one's hat, to
To declare one’s readiness to consume one’s headgear if a statement should prove false, an event should not occur, and so on. The likelihood of actually doing so is presumably very remote, which is the very analogy being drawn (to a statement’s being false, an event not occurring, and so on). The expression appeared in Dickens’s Pickwick Papers (1836), in the words of one clerical gentleman, “Well if I knew as little of life as that, I’d eat my hat and swallow the buckle whole.”
See also: eat