Dear SS, The speaker does not doubt that it will go well. He is prepared therefore to «eat his hat» if it does not go well. The absurdity of the offer accords therefore with the absurdity of the possibility.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
CalifJimGoogle:Actually Google wouldn't give good answers. This was used in literature first by Charles Dickens in "Pickw.ick Papers" (1837) : "If I knew as little of life as that, I'd eat my hat and swallow the buckle whole". The OED suggests the original form was "I'll eat an old Rowley's hat" but gives no further information.
"eat my hat" origin idiom