go and boil your head

ガリバー旅行記、アリスの不思議の国、ドラエもんを混ぜて作ったようなChocolate Factoryからの引用もこれが最後になります。 Square Candies That Look Roundという奇妙なキャンディの話の個所からです。
"They look round," insisted Mr. Wonka.
"They most certainly do not look round!" cried Veruca Salt.
"Veruca, darling," said Mrs. Salt, "pay no attention to Mr. Wonka! He's lying to you!"
"My dear old fish," said Mr. Wonka, "go and boil your head!"
状況からすると、「頭を冷やせ」とMr. Wonkaが言ってもおかしくない場面ですが、 "go and boil your head" は少なくとも表面的にはその反対の事を言っています。　いづれにせよイデオムのようなので、辞書で調べます。
・Urban Dictionary: An insult, roughly equivalent to get lost or in some cases go fuck yourself. As far as I can tell, it is mainly used in Britain.: Oh, go boil your head, you fermented bastard.
この他の普通の辞書にはこのフレーズの説明は見あたりませんでしがが、次ぎのサイトに詳しい説明がありました。
・Phrase Finder: An insult, of the form go and do something bad for yourself. Other examples are, go and play in the traffic and the more recent, and vehement, eat shit and die.
This could be related to the fact that in South Pacific societies cooked food was 'noa' or the opposite of tapu (tapu is loosely translated as 'sacred', although it has a deeper meaning than that). The head was considered the most sacred part of the body and one doesn't usually pat children on the head, or stroke a person's hair. An exhortation to turn your head into cooked food was regarded as the greatest possible insult.
The earliest citations of the phrase in print in English don't exactly support that derivation. The original meaning of the phrase was rather milder than it is now and relate to people who were told to stop promoting silly opinions, i.e. don't be so stupid. These example date from the 1930s. In the UK there's a version of it in Compton MacKenzie's satire on the British intelligence services, Water on the Brain, 1933: "Go away and boil yourself"
「頭を冷やせ」などと言う冷静な言い方ではなく、「馬鹿を言え！」とか、日本語ではあまりUrban Dictionaryの説明に出てくるような品のない言い方は普通はしないので、対応する日本語が浮かびませんが（時々失敗した時に「糞！」って言いますが）、相手を侮辱する表現なのですね。