eat one's hat, to

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.

Declare one's certainty that something will not happen or is untrue. This hyperbolic expression almost always follows an if-clause, as in If he's on time, I'll eat my hat, that is, "I'll consume my headgear if I'm wrong." Charles Dickens used it in Pickwick Papers (1837): "If I knew as little of life as that, I'd eat my hat and swallow the buckle whole." [First half of 1800s]

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.”