'I'll eat my hat' is an expression people use when they think something is very unlikely to happen.
If Arsenal win the Champions League, I'll eat my hat!
I don't think I'll make it past the interview stage for this job. If I do, I'll eat my hat!
If the government survives this latest scandal, I'll eat my hat!
Don't confuse it with
If you are forced to 'eat your words', you admit that you are wrong about something you have said.
I advised Suzanne against setting up her own business, because I didn't think it would be successful. Well, I've had to eat my words – she's now a multi-millionaire!
Watermelons are thought to have originated in southern Africa, where they can be found growing wild. Cricket fans wore hats carved out of watermelons during the second Test match between South Africa and Australia this week.