Eat Your Hat
You're telling someone you hate them so much you'd let them eat your hat and you wouldn't even blink. It's like telling them you'd let them die in a hole.
"Eat your hat, you piece of garbage! I hate you!", after your friend broke your phone.
"Eat your hat, you stupid jerk! I hate you!", after your brother took your last toy.
"Eat your hat, you idiot! I hate you!", after your cousin ate your lunch.