We often use the phrase “What on Earth…?” and a similar phrase, “What in the world…?,” when we want to express surprise, confusion, or anger. The Earth is the planet that humans live on, but this expression has nothing to do with the planet. You can use it alone (“What on Earth?”) or as the beginning of a longer sentence (“What on Earth do you mean?”)
Let’s say (let’s use as an example) that your co-worker comes to work wearing shorts and a T-shirt, when he normally wears a business suit. You may ask him, “What on Earth are you wearing that for?” or “Why in the world are you wearing that?” You’re expressing your surprise and confusion, and you’re asking for an explanation.
You can use all of the following, depending on what you want to express:
“What on Earth…?” Example: “What on Earth was I thinking when I quit my job?”
“Why on Earth…?” Example: “Why on Earth doesn’t Jeff have any hair?”
“How on Earth…?” Example: “How on Earth did you drive from New York to California in three days?”
“Who on Earth…?” Example: “Who on Earth put a boat in the swimming pool?”
“Where on Earth…?” Example: “Where on Earth are you going to buy a doghouse large enough for your dog?”
“When on Earth…?” Example: “When on Earth are you going to have time to learn to play the piano? You work 80 hours a week!”
Let’s take another example: It’s 3:00 in the morning and you’re sleeping when your friends knock on your door, laughing and making a lot of noise. When you open the door, you may say, “What on Earth do you guys want at this hour?” You’re asking them to explain themselves and at the same time, expressing your displeasure. Actually, depending on how unhappy you are with your friend, you would probably use the phrase “What the hell…?” instead of “What on Earth…?” “What the hell…?” is used in the same way, but expresses a much stronger feeling and is more likely to be used to express anger, rather than surprise or confusion. Hell is the place where Christians and others believe bad people go after they die. “What the hell…?” is commonly used today, more so than “What on Earth…?,” which is considered a little old-fashioned.
When people are extremely (very) angry, they may use the phrase, “What the f__k?” I don’t want to include an offensive word here in the blog, but I think everyone knows the four-letter “f-word” in English that is very offensive. People use this when they are very upset, or sometimes jokingly with their close friends. You might say, “Why the f__k did you have a wild party in my apartment when I was out of town?! As no doubt you know, the four-letter “f-word” is one of the most offensive and insulting words in English and shouldn’t be used unless you are talking to very good friends, or if you want a punch in the nose.
Thanks, Andrea, for this question.